Viruses are nearly always bad news, but sometimes they can surprise you. The tobacco mild green mosaic virus is one of those.

This viral disease can infect your peppers, potatoes, and tomatoes. It can also help out.

Symptoms of tobacco mild green mosaic

Stunting, leaf mottling and puckering, and deformed fruits are all symptoms of tobacco mild green mosaic. These symptoms look similar to cucumber green mottle mosaic, tobacco mosaic, and tomato brown rugose, and with good reason. The viruses that cause these diseases are close cousins.

There are no chemical treatments for tobacco mild green mosaic. Good cultural practices like hand washing, sanitizing your tools, and avoiding infected areas, can slow the spread of this disease. And keep weeds in the nightshade family at a distance.

Tobacco mild green mosaic trivia

This virus is an approved herbicide in the US. Scientists discovered that the virus responsible for tobacco mild green mosaic is lethal to tropical soda apple. Tropical soda apple (Solanum viarum Dunal) is a spiny, invasive weed that makes farmland, orchards, and pastures less productive. Florida estimated, in 2022, that tropical soda apple infested 1 million acres of their agricultural land. 

According to the American Chemical Society, this virus can kill parasitic nematodes. In 2017, parasitic nematodes cost the global agricultural industry $157 billion. That’s a lot of zucchini!

So maybe the tobacco mild green mosaic virus isn’t all bad.

Researching yesterday’s post on bacterial blight, I was astounded at the number of diseases caused by Pseudomonas. Pseudomonas [soo-doh-MO-nas] is a genus of bacteria that most of us gardeners end up fighting.

These bacteria are commonly found in plant debris, soil, and water. They also hide out in many  dicot seeds. But don’t worry, Pseudomonas only infects plants with leaves and stems. The rest of your garden is safe. *wink*

Pseudomonas plant pathogens

To date, more than 500 strains of Pseudomonas have been sequenced. Here is a list of the most common bacterial diseases caused by Pseudomonas:

• Angular leafspot
• Bacterial blight
• Bacterial brown spot
• Bacterial canker
• Bacterial dieback
• Bacterial leaf spot
• Bacterial soft rot
• Bacterial speck
• Citrus blast
• Halo blight
• Leaf spot
• Olive knot

 
Small dark spots appear and expand into odd-shaped dead areas in nearly all these diseases. It’s all downhill from there.

Pseudomonas isn’t all bad

As handy as it would be to say that all Pseudomonas are evil, it ends up that some of these soil bacteria help plants stay healthy. In fact, they practically make life possible on Earth. Life sure can be messy, eh?

Some Pseudomonas protect plant roots against disease-causing Fusarium fungi and Pythium oomycetes. They also protect against plant-eating nematodes. And other strains help activate disease resistance within wheat and other cereal crops. Some Pseudomonas can metabolize pollutants and are used in bioremediation.

Finally, Pseudomonas helps form most snowflakes and raindrops that fall on Earth.

Now you know.

Bacterial blight is not the same thing as common bacterial blight, which attacks legumes. To make matters worse, many people call another bean variety of blight ‘bacterial blight’. I know, it gets confusing. Let’s see if we can clarify some of this.

The cause of bacterial blight

Bacterial blight, also known as blossom blight or shoot blight, is caused by a bacterium called Pseudomonas syringae. There are over 50 different strains of this bacteria that cause plant disease. Common examples include bacterial canker, bacterial speck, citrus blast, and halo blight. Pseudomonas syringae is commonly found on the exterior surfaces of healthy plants. It is only when bacteria get inside, through wounds or natural openings, that the trouble starts.

Bacterial blight symptoms

Like the other diseases caused by this pathogen, the first symptom is water-soaked areas on leaves. These areas turn brown and mushy and often have yellow halos. If these spots occur early in a leaf’s development, leaf curling and twisting may also occur. Leaves may also start dying from the outer edge, with the infection moving inward toward the center. Twigs may exhibit black streaks, and it is common for infected blossoms, branch tips, and leaves to die. If infected twigs develop a shepherd’s crook shape, it’s probably fireblight.

Managing bacterial blight

Pseudomonas syringae is most commonly spread by wind and rain. Insects and your garden tools can also be part of the problem. You have to assume that the disease is present. These bacteria can survive in diseased plants, infected plant debris, and soil.

Once a plant is infected, you can try to save it by trimming 10 to 12 inches below infected areas, making sure to disinfect your pruners between each cut. You can dip them in a 10% bleach solution for 30 seconds, though bleach is hard on tools. You can also use bathroom cleaner or other spray disinfectants that contain at least 70% alcohol. Just be sure to give it a few minutes to work before making another cut. Infected plant material should be bagged and thrown in the trash right away.

Prevention is easier. To prevent bacterial blight from taking hold in your landscape, use these tips:

• Start with resistant varieties.
• Space plants out enough that they get some good airflow.
• Feed and water plants appropriately to help them stay healthy.
• Avoid overhead watering
• Do not work around plants when they are wet.

Fixed copper sprays may also help.

Also known as cherry X disease, peach X disease, and cherry buckskin disease, X disease makes fruit small, pale, and bitter. It can affect stone fruits, such as apricots and cherries. Once a tree is infected with X disease, it must be removed to prevent the infection from spreading to other trees.

X disease is often mistaken for root rot. To figure out which it is, look closely at the graft union. That’s where the rootstock was grafted onto the fruiting stock. Pits and grooves are visible near the graft union if it is X disease. Cut into the wood and see brown areas in the phloem. These other symptoms of X disease are also common:

• Chlorosis
• Purple leaf spots that create a “shot hole” effect
• Sparse leaf cover
• Tip dieback
• Upward leaf curling

The fruit from infected trees tends to be pale, pointed, small, and nasty.

X disease hosts

There are two types of X disease hosts: those that don’t mind the infection (reservoir hosts) and those who succumb to the disease (non-reservoir hosts). Reservoir hosts are a problem because they look perfectly healthy but provide a source of infection to many important fruit and nut trees in our landscapes. Almond trees, chokecherries, sweet and sour cherries, Japanese plums, clovers, and dandelions are all reservoir hosts. [Dandelions?!!? Yes, dandelions.]

Nectarines and peaches can also become infected with X disease but are non-reservoir hosts. This means that they can catch the disease but not spread it. Scientists don’t yet understand how that happens, but they’re working on it.

X disease management
 
Since scientists have been unable to reproduce the phytoplasmas responsible for this disease in the lab, there are currently no treatments for this fatal disease. To reduce the likelihood of X disease occurring in your landscape, do your best to manage the insect pests responsible for carrying the pathogen.

The insects that spread X disease are often found on beets, burclover, ceanothus, curly dock, hawthorn, and pyracantha. Because of this, it is a good idea to maintain some distance between these plants and vulnerable trees.

If you suspect X disease has infected one of your trees, get help. Contact your local Master Gardeners or the Department of Agriculture for verification. They can also help you figure out the safe removal and disposal of infected trees.  

Cherry crinkle symptoms

Vein clearing, especially in the margins, is the most common symptom of cherry crinkle. Vein clearing refers to the way veins look lighter or more yellow than normal. I wasn’t able to find a photo that I could use, but these other symptoms may also occur:

• Leaves are thinner.
• Leaves become more heavily serrated or irregular.
• Slot-shaped perforations may appear.
• Affected leaves often fold in half lengthwise, wilt, and drop in midsummer.
• Blisters may appear on the underside of leaf veins.
• Silvering may occur on upper leaf surfaces.
• Buds may be distorted.
• Flowers are abundant, but very little fruit develops.
• Fruits tend to be pointy and flattened on one side, with a pointed ridge.

Cherry crinkle management

If you suspect cherry crinkle, send a sample of soil to a lab for testing. This will tell you if your boron levels are low. It’s not a good idea to add nutrients without a soil test because too much of a nutrient can cause just as many if not more, problems as a nutrient deficiency.

If boron isn’t the problem, the tree is probably a mutant and should be replaced.

Cherry crinkle can be spread to unaffected trees by grafting mutated scions onto healthy wood. When shopping for a cherry tree, try to find one listed as unlikely to crinkle.

Blackheart is a disorder caused by environmental conditions. It occurs in celery, potatoes, sweet potatoes, tomatoes, and other garden crops. The causes of blackheart are the same. If seen in almonds, it’s a fungal disease called Verticillium wilt.

Causes of blackheart

Blackheart is the result of insufficient oxygen or too much CO2. How can you control those levels, you might ask. It’s easy. Picture this: it’s a hot day. Your plants are wilting. You add water, a lot of water. Your intentions are good, but now the ground is saturated. This localized flood pushes oxygen out of the macropores and micropores of the soil, and blackheart takes hold. Blackheart is also caused by too many cold days, like in your refrigerator. 

Blackheart of celery

If young tissues in the middle of your celery plants turn black, it may be blackheart. Much like blossom end rot of tomatoes and tip burn of lettuce, this condition is a calcium uptake problem. It means plants cannot access enough of it. It may be a calcium soil deficiency or insufficient water. An inexpensive lab-based soil test is the only way to know what is in your soil. Irregular watering, soil salinity, and excessive fertilizer, specifically magnesium, nitrogen, or potassium, can interfere with calcium uptake.

Sometimes things look fine on the outside when the inside is damaged.

If you cut a beet, potato, seed, or watermelon in half and find a hole in the center, it has hollow heart. Also known as hallowheart, this condition indicates that the plant faced more stress than it could handle.

Hallowheart of watermelons

Hollow heart in watermelons is a pollination problem. Poor pollination occurs when temperatures are low or when there are not enough pollinators in the area. Somehow, poor pollination causes the interior of watermelon to develop triangular, often symmetrical gaps in the fruit. Seedless watermelons are more susceptible to hollow heart than seeded varieties, and orange and yellow-fleshed watermelons are the most likely to develop this condition. Improve pollination rates in your garden by attracting pollinators with flowers that appeal to honey bees and bumblebees. You can offset low temperatures with row covers.

Hollow heart in watermelons can also occur when they get too much fertilizer and insufficient water. The cells responsible for producing fruit cannot keep up with the cells producing rind under those conditions.
​
Hollow heart of beets

If you cut into a beet and find an empty center, it’s time to send a soil sample to a lab for testing. Hollow heart in beets is commonly a symptom of a boron deficiency. Look closely at the petioles (leaf stems) for signs of cracking, abnormalities, or stunting. If the cavity is rotting, it may be blackheart.

Hollow heart of potatoes

Hollow heart in potatoes can be a weather problem or a water problem. Potatoes are greedy plants. They need a lot of water and nutrients to produce those fat tubers. Hollow heart occurs when a developing potato is hit with cold weather, followed by hot weather. Or when a sudden dry spell ends with an extended wet period. Like that one time you forgot to water them and then gave them extra water to make up the difference.

​Where the cavity occurs will tell you its cause. Water or nutrient stresses that occur later in the growing season create holes at the bud end of the tuber. These cavities do not have a brown center. When hollow heart appears at the stem end of a tuber, cold weather followed by hot weather is the culprit. Brown centers are a common symptom of stem end hollow heart.

Potato virus Y (PVY) is the Big Daddy of potato problems, right up there with early blight. And potatoes aren’t the only plants at risk. Peppers, tomatoes, and groundcherries can all catch PVY. Losses can be as high as 80%. Infected potatoes that make it to harvest don’t last in storage. And who wants to eat an infected potato?

Potatoes come in many colors: blue, purple, red, yellow, and white. But why does potato skin turn green sometimes, and can it hurt you?

The simple answer is yes. Green-skinned potatoes can make you sick. According to WebMD, you can peel green-skinned potatoes, but those potatoes are still not entirely safe to eat because of toxins.

Potato toxins

Like other members of the nightshade family, potatoes produce toxins. The toxins produced by potatoes are called solanine and chaconine. These toxins are defense mechanisms produced in abundance when the tubers are dug up and exposed to light. The green color under potato skin is chlorophyll. You can use it as a signal that lets you know a spud’s chemistry has changed.

Why do potato skins turn green?

Tubers belong underground. Uninjured potatoes are relatively stable in cool, dark locations. Expose them to light once, and nothing happens. Expose them to light several times, and things start happening. Imagine, if you will: First, they get dug up at the farm and see daylight for the first time. Then they are moved to a shipping truck and rolled down the freeway for more sunlight. Drop them off at the processing plant, and they get more light. You get the idea. By the time they leave the grocery store (where they got even more light) and arrive at your kitchen, some potatoes will have shifted from storage to growth mode. That is when trouble starts.

Is peeling enough?

Many say that peeling the green skin away makes the spud safe to eat, which is not entirely accurate. While most potato toxins are in the skin, they are also present in the rest of the potato. [Just as moldy cheese has mold growing throughout its interior, even if it is invisible.] Peeling green-skinned potatoes may not be enough, and cooking them does not affect the toxins.

Healthy individuals with good digestive systems are generally not harmed by occasional green-skinned potatoes. If you notice any of these symptoms, however, see a doctor:

• Abdominal pain
• Diarrhea
• Feeling out of sorts or confused
• Fever
• Flushed skin
• Headache
• Vomiting

Preventing green-skinned potatoes

You can’t control what happens to your potatoes before they arrive at your home (assuming you haven’t started growing your own yet). You can store your potatoes in a dry, chilly location with as much darkness as possible. Your refrigerator or pantry are ideal.

If an occasional green-skinned potato appears, throw it in the trash if it was from the store. If homegrown, add it to the compost pile or plant it.

Powdery scab may sound like an adolescent skin problem, but it is not. It is a potato disease that leaves spuds looking, well, scabby. You can cut away the affected bits and eat the rest of your potato, but the skin is where most of a spud’s nutrients are stored, so that’s a shame. 

Also known as corky scab, let’s see what we can learn about this disease and how to prevent it.

How to prevent powdery scab

Infected potatoes are not the only way powdery scab can enter your garden. Other potential pathways include infected bulbs, corms, rhizomes, firewood, planting containers, shoes and clothing, and soil from introduced plants.

I am a big proponent of buying used garden gear from yard sales and thrift stores. But you have to wash everything thoroughly before using it. And always put new plants into quarantine.

Chemical controls are not effective against powdery scab. But these practices can help prevent powdery scab:

• Install certified pest- and disease-free eye potatoes.
• Provide good drainage.
• Let the soil dry out between waterings.
• Plant resistant cultivars.
• Use the least amount of nitrogen needed.

Whatever you do, do not give in to the temptation to install spuds and other vegetables from the grocery store. These plant sources may be convenient and inexpensive, but they can bring a world of hurt to your garden. Produce bought at grocery stores is certified safe to eat, but it can still carry soil-borne diseases that may linger in your soil for decades. If you are growing in containers, this is less of a problem. If any disease appears in that case, toss it in the trash and start over. Hopefully, you catch it before any problems spread to the rest of your garden.

While you may not be able to eliminate the oomycetes that cause avocado root rot from your property, you can control the conditions that allow them to cause harm.

Measles on fruit, leaves, or stems can mean many different things, but it is not the human measles virus.
  
Measles on plants can indicate fungal diseases that may look like anthracnose. Measles can also be a symptom of nutrient toxicities or an overabundance of irrigation water.

Grape measles

If it looks as though your grapes have measles, it may be black measles, also known as esca or Spanish measles. Grapes infected with black measles develop small, reddish-brown spots on the fruit. These spots may appear at any time during the growing season. Ultimately they will cover the fruit, causing it to turn black and shrivel. If you were to eat these grapes, they would taste bitter. Leaves develop a characteristic ‘tiger stripe’ pattern in which the veins of white grapes remain green, haloed with yellow, and surrounded with dead, brown tissue. Red and purple cultivars develop reddish areas instead of yellow. Other symptoms include shoot tip dieback and complete defoliation. Also, cut wood tends to ooze a dark sap, and cross-sections show dark streaking in the xylem. Infected plants are highly susceptible to other fungal diseases, particularly rots.​

Healthy soil is teeming with microscopic life. Most soil organisms are beneficial, but some of them carry diseases.

Phytomyxea

There is another class of soil-borne disease carriers called Phytomyxea [FI-toe-muh-kia]. Scientists used to think they were a type of slime mold, but genetic testing and electron microscopes have taught us that they are a unique group. Phytomyxea are plant parasites that can cause clubroot in cruciferous vegetables and powdery scab in potatoes.

Viral diseases

Soil-borne viral diseases are rare. In most cases, nematodes and certain fungi carry these pathogens. Lettuce necrotic stunt and wheat mosaic, which causes stunting and mosaics in wheat, barley, and rye, are two common soil-borne viral diseases.

How to prevent soil-borne disease

In nature, plant diseases rarely get out of hand. Plants’ defense mechanisms and other organisms keep most pathogens in check. But, when we select plants, spray chemicals, and disturb the soil, we interrupt those natural processes. The primary cause of soil-borne diseases taking hold is an imbalance in soil populations. Reduced biodiversity gives pathogens the upper hand.

One way to reintroduce that biodiversity is by top dressing with aged compost. Research has shown that top dressing with compost is very effective at suppressing soil-borne diseases in greenhouses, though less so in the field. There is a direct correlation between how much compost was applied and its effectiveness. Interestingly enough, sterilized compost is less effective. I think we can assume the effect is at least partially biological.

​As with most diseases, three factors must be present for a problem to occur: the host plant, the pathogen, and the right environmental conditions. Remove any one side of the disease triangle, and the disease is prevented or controlled. Crop rotation is an excellent way to break this disease triangle. Your rotation schedule will vary depending on the plants and pathogens in your garden.

While you can sometimes apply treatments directed toward specific pathogens, they don’t always work. Most of these treatments consist of other microorganisms that prey on the pathogens. These only work if your soil already has everything the introduced microorganisms need. The funny thing is, if all those things were already there, so would the predators. Biodiversity. Mycorrhizal fungi (good guys) often create protective mats containing antibiotics and pathogenic toxins around plant roots. They also help plants absorb nutrients.

Use these tips to prevent soil-borne diseases in your garden:

• Do not plant grocery-store produce or live herbs from the grocery store.
• Use disease-resistant varieties and certified disease-free seeds and seedlings.
• Select plants suited to your microclimate and soil pH.
• Avoid damaging root systems and employ no-dig gardening.
• Improve drainage.
• Regularly test soil to help you maintain proper nutrient levels.
• Monitor plants frequently for signs of disease.
• Treat fungal diseases with fixed copper or sulfur according to the plant species.
• Remove and dispose of infected plant material.
• Keep host weeds at a distance.
• Rotate crops.
• Sanitize tools, planter pots, and trellising between uses.
• Avoid walking through areas affected by diseases.

Finally, as tempting as they may be, chemical treatments are rarely a good choice for backyard gardeners. Pathogens are developing resistance to these treatments. Increasingly powerful chemicals are applied, resulting in a dangerous escalation. Whenever possible, use some other method of controlling soil-borne diseases.

Why do soil bacteria matter to gardeners?

Most soil bacteria are valuable members of your team. They provide a huge benefit to your soil and plants. And you need to know what the bad bacteria look like when they start to set up housekeeping. The earlier you break those disease triangles, the faster your can return to harvesting your delicious crops.

Most bacteria are aerobic, which means they need oxygen. This is why turning your compost pile makes everything decompose faster. You are providing the decomposer bacteria with the air they need. If you don’t, the anaerobic, non-air breathing bacteria take over. Those are the ones associated with rot and purification. [Ew!]

Did you know that soil bacteria will consume more water than they can hold, causing their bodies to burst? Yet another argument against over-watering...

In the garden, bicarbonates are touted as cure-all treatments of powdery mildew, gray mold, septoria leaf spot, and other fungal diseases, particularly sodium bicarbonate (baking soda) and potassium bicarbonate. The truth is, baking soda is a type of salt and always a bad idea in the garden. Potassium bicarbonate, on the other hand, is an effective organic fungicide. 

But what about ammonium bicarbonate? What are bicarbonates, anyway?

Bicarbonates

In chemistry, the word ‘bicarbonate’ is inaccurate and outdated. It was first coined in 1814 by a chemist who observed that there is twice as much carbon as sodium in sodium bicarbonate. After different types of bicarbonates were identified, with different ratios, the observation became irrelevant, but the habit lives on.

Bicarbonates are the main form of dissolved inorganic carbon in the ocean. In freshwater plants, bicarbonates are released into the water as part of photosynthesis. This can shift the water’s pH to toxic alkaline levels. This continues until nightfall, when photosynthesis stops and respiration releases carbon dioxide, causing pH to return to normal. Bicarbonates commonly act as pH buffers in the human body [plop, plop, fizz, fizz] and in soil. 

Ammonium bicarbonate fertilizer

In the plant world, ammonia means nitrogen. This makes ammonia bicarbonate sound like a good idea as a fertilizer, right? In China, ammonium bicarbonate is used as an inexpensive fertilizer. But, because of its instability, it is being phased out in favor of urea. Also, ammonium bicarbonate is an eye, skin, and lung irritant. If you were to use it (against my advice) be sure to wear protective clothing and a respirator. 

High soil bicarbonate levels commonly occur when soil or irrigation water have a pH of 7.5 or higher. Alkaline soil and irrigation water tend to have lots of bicarbonate and carbonate ions floating around. These ions tend to attach themselves to and transform calcium and magnesium into less soluble forms that are difficult for plants to use. Also, as these minerals are altered, they leave salt behind in your soil. Not good. When these conditions occur, chelated fertilizers should be avoided. The level of bicarbonates in your soil also determines how much acid is needed to acidify the soil. 

If powdery mildew or other fungal diseases are causing problems in your garden, forget the baking soda and ammonium bicarbonate, Instead, space and prune plants for better air flow and apply potassium bicarbonate, Bordeaux mixture, sulfur, fixed copper, or milk. You may also want to apply insecticidal soap (not dish soap) to reduce the spread of disease by ants.

Conks are woody, shelf-like structures produced by some fungi. These fruiting bodies are often seen on trees and can indicate fungal diseases, such as canker rot or butt rot.

Can you see a crack in the trunk or branches of your tree? It may be canker rot.

Canker rot is a collection of fungal diseases that eat away at the interior of tree trunks and branches, weakening the tree and setting the stage for other pests and diseases. Canker rots can also girdle your tree and kill it. While most commonly seen in ornamental trees, canker rot can occur in apple and other fruit and nut trees.

Canker rots can also cause swelling, sunken areas, gnarled bark, and conks. Conks are shelf-shaped fungal fruiting bodies. After spores are released, the conk will dry out and darken. It may remain on the tree or fall off.

If you were to see inside your tree, you would see that the heartwood and sapwood have become discolored. Instead of warm, rich yellowish-browns of healthy wood, you would see gray, orange, or even pink-tinged wood, often extending three or more feet beyond the canker.

Canker rot lifecycle

The fungi responsible for canker rot usually enter trees through pruning cuts and wounds. Fungi attach to the wood and then move to the cambium to access the water and nutrients flowing through the vascular bundle. The fungi also move to the bark, where they eject spores carried by wind to nearby trees.

How to control canker rot

As always, healthy trees are better able to protect themselves. So, select species suitable to your microclimate, plant them at the proper depth, irrigate and fertilize your trees properly, and monitor for signs of problems. Other actions you can take to reduce the chance to canker rot occurring in your trees include:

• Make sure pruning cuts are clean and do not use sealants.
• Whitewash exposed trunks and limbs to prevent sunburn damage.
• Remove tree supports as soon as they are not needed.​​

​Trees with canker rot may fall over at any time. Large trees weigh several tons and can be extremely dangerous. If you suspect canker rot, call a licensed arborist right away.

Not to be confused with the other black spot (Diplocarpon rosae), which primarily affects leaves, Ascochyta blight can infect any aboveground portion of your chickpea plants, as well as your lawn. Lawns infected with Ascochyta blight suddenly develop brown patches of dead-looking grass. Ascochyta blight is caused by Ascochyta rabiei (formerly known as Phoma rabiei).

Ascochyta blight symptoms

Brown lesions that start at the base of seedlings may start out looking like damping-off disease, but these lesions continue to move up the plant, eventually affecting everything aboveground. ​

Ascochyta blight management

Ascochyta blight can be spread by infected seeds, so always start your garbanzo bean crop with certified disease-free seeds. Do not use that bag of garbanzo beans from the grocery store. The price might be appealing and those seeds are safe to eat, but they may also carry any number of pests and diseases that might take years to get rid of. You can also select disease resistant varieties. According to UCANR, the following varieties of garbanzo are currently resistant to Ascochyta blight: Sierra, Dylan, Sutter, San Joaquin, and the Airway Farms (AWF) series. That resistance can and will change because fungi evolve faster than plants.

At the first sign of infection, the affected plant should be removed and tossed in the trash. You don’t want to leave infected plant material in the garden or compost pile because this can simply spread the disease to more plants. Ascochyta blight does not survive in the soil, so crop rotation is a good way to break the disease cycle. 

If Ascochyta blight has been a serious problem in past years, space plants out more for better air flow, and plant seeds as late in the season as possible.

Fungicides can be used at the first sign of disease and reapplied according to package directions.

Generally speaking, milk or whey, applied before exposure to powdery mildew does reduce disease incidence. This may be because the benign microorganisms that start growing on the milk make it more difficult for powdery mildew organisms to take hold. Another theory is that the fatty acids contained in milk have antifungal properties. It may be for another reason altogether. We don’t know yet. Note that none of these studies succeeded using nonfat milk. 

Research has also shown that foliar sprays of milk are effective in treating viral diseases, particularly tobacco mosaic and other mosaic diseases on barley, beans, beets, celery, peas, spinach, sunflowers, tomatoes, and zucchini. It is believed that the milk may either deactivate the viruses, or physically isolate them, but no one knows for sure. The milk spray may also prevent aphid attack, thereby reducing the number of aphid-borne viral diseases.

Problems with milk in the garden

The main problem associated with milk in the garden is that there has been very little scientific research conducted. Most of the information available is anecdotal, at best. Besides being expensive, there are three problems you should keep in mind before deciding to use milk in the garden:

1. Rotten milk smells bad.
2. Dried skim milk, in particular, has been shown to induce, rather than prevent, Alternaria leaf spot, black rot, and soft rot on cruciferous crops.
3. Benign fungal organisms that grow on the milk can make your plants unattractive.

How to apply milk

If you decide to use milk to prevent disease, it is recommended that whole milk be sprayed on soil prior to planting and again when insects appear or just prior to when powdery mildew and similar diseases are expected to occur. You can also dump sour milk into your compost pile or around acid-loving plants, such as blueberries.

So, can milk help improve your plant and soil health? Yes, and no. Used as a preventative antifungal, antiviral, antibacterial treatment, milk may reduce disease incidence by 50% to 70% on some plant species.

Small black spots on tomatoes and tomato leaves often indicate bacterial spot.

​Those black spots might not look significant, but this bacterial disease can also affect peppers, eggplant, groundcherries, and tomatillos. A close cousin to the bacterial spot of almonds and practically impossible to differentiate from bacterial speck without a microscope, bacterial spot (Xanthomonas campestris pv. vesicatoria) is perfectly capable of killing your tomato plants.​ 

Bacterial spot symptoms

Symptoms of bacterial spot can appear at any time during a plant's development, from seedling to mature plant and on all aboveground plant parts. Bacteria enter through wounds and stomas. Infected younger plants can lose all their leaves. Older plants exhibit insignificant-looking water-soaked areas on mature leaves, usually near the bottom of the plant, making it easy to dismiss this disease. But that would be a mistake.

These lesions start yellow or light green, turning dark brown or black. Older spots may become raised areas that average 1/3" across. Larger damaged areas may occur at the margins (leaf edges).

Immature fruit can also be affected by bacterial spot. Bacteria enter through tiny hairs called trichomes. Infected areas have tiny, raised black dots that become sunken or dimpled and surrounded by a white halo, similar to bacterial canker. The halos eventually disappear as the spots get larger and become scabby. If these fruits mature before rotting on the vine, they are still edible. Just cut out the diseased areas and dispose of the infected parts in the trash. Do not add infected plant material to your compost pile.

Controlling bacterial spot

Because these bacteria overwinter in infected plant debris, you can protect next year's crops by clearing infected plant tissue out of your garden each fall. The disease can also appear on volunteer tomato plants, so watch rogue plants closely for signs of infection. Splashing  rain, irrigation water, and contaminated tools can also spread the disease, so avoid overhead watering and sanitize your garden tools regularly.

Since these bacteria need humidity and water droplets to survive, pruning for good airflow can go a long way toward preventing this disease.

If you are like me and save seeds from each year's crops, be sure you don't use those from infected plants, as you will perpetuate the disease. As always, only buy certified disease-free plants and seeds and quarantine new plants. Fixed copper sprays may help reduce the chance of bacterial spot. There have been some cases of copper-resistant bacteria, so it's hard to say.  Crop rotation  can also help break this disease triangle.

Protect next year's crops by tossing plants infected with bacterial spot into the trash and providing good airflow around future plants.

If you grow raspberries, you should be on the lookout for spur blight. 

Blackberries and other bramble fruit are not affected by the Didymella applanata fungi, but red raspberries are especially vulnerable.

Currently found most often in Scotland, Oregon, and Washington, spur blight can significantly reduce your raspberry crop.

Look closely to see tiny black dots emerging from those cracks. Those dots are fungal fruiting bodies called pycnidia. If you look at pycnidia with a microscope, they are flask-shaped. A different type of fruiting body, perithecia, comes next. Perithecia are also black, but they are medium-sized and, if you watch, erupt with spores.

Spur blight lifecycle

Spur blight spores travel in wind, rain, and irrigation water. These spores are released each time wet weather occurs, even if that wetness is your garden hose. Infected water that lands on or near young canes, newly forming buds, leaves, wounds, or stomas, can result in infection. Once inside, this disease spreads throughout the plant, overwintering inside the lesions.

How to manage spur blight

Proper sanitation and pruning methods will go a long way toward preventing spur blight on your beloved raspberries. In late winter or early spring, before new canes emerge, remove all dead, diseased, or weak canes and put them in the trash rather than the compost pile. 

Keep canes properly spaced for good airflow, and train them up trellising to allow canes to dry quickly. It makes watering your raspberry plants at ground level easier, too. Keep weeds away, as they compete for water and nutrients and reduce airflow around the canes. Unless a lab-based soil test has indicated a need for fertilizer, avoid feeding your raspberry plants when spur blight is a problem. The presence of too many nutrients causes plants to produce an abundance of vulnerable tissue.

As always, only buy certified, disease-free plants and place new plants in quarantine. In the case of severe infection, fixed copper or lime sulfur treatments may reduce lesion size and control internal infection, but only if applied when new shoots are 8-10” long.

Spur blight is easier to prevent than treat. And raspberries are worth the effort.

Strawberry vein banding is an invisible viral disease spread by strawberry aphids. You will never know it has infected your plants until another virus infects your strawberries.

As soon as another virus infects your plants, usually strawberry crinkle, the leaf veins of your strawberry plants suddenly start to turn yellow. And if the strawberry mottle virus comes along, those yellowing veins won’t be visible. This triple threat is called strawberry decline.

Symptoms of strawberry vein banding

The leaves of infected plants tend to be significantly smaller. Vein yellowing, when visible, appears erratically in new growth first. Sometimes only part of a vein has turned yellow. The two halves of each unopened leaf may look closer together than is normal, and the margins, or leaf edges, are wavier than normal. Some crinkling of the leaf surface may also occur. As the leaf opens, the bands of yellow become a little more obvious.

Symptoms appear more strongly in the second and third leaves but are not likely in later growth. Unfortunately, the other symptoms include stunting and reduced fruit and runner production. Vein banding can reduce strawberry crops by nearly 20%. If another virus takes hold, you can lose your crop entirely.

Strawberry vein banding vectors

Strawberry aphids and other aphid species carry strawberry vein banding to your plants. Grafts  from infected plants also spread the disease. Strangely enough, dodder can also spread the disease, but the sap from an infected plant cannot. Stranger still, a clone of the vein banding virus can infect turnips, a completely unrelated species.

Strawberry vein banding control

In a word, you can’t. Strawberry vein banding can only be prevented by installing certified disease-free plants, placing plants in quarantine when they first arrive, and removing any infected plants.

Since aphids can fly at points in their development, the threat of this and other viruses is constant. All you can do, besides the preventive measures listed above, is monitor your plants for signs of aphids and control them as well as you can. Insecticides and  insecticidal soaps work against aphids, but your strawberry plants need honey bees and other pollinators to produce fruit. Those insecticidal controls will impact your helpers, too, so avoid them while plants are flowering.

Closely monitoring your strawberry plants and keeping other plants that might host aphids at a distance can go a long way toward preventing vein banding in your garden.

Strawberry pallidosis is one of several viruses that make up strawberry virus decline.

Infected with only one of these diseases, strawberry plants often remain symptomless. It isn’t until a second virus enters the game that symptoms begin to appear. These other viral diseases include strawberry vein banding, crinkle, mottle, mild yellow edge, and beet pseudo yellows.

Symptoms of strawberry pallidosis

Stunting, reduced fruit and runner production, and older leaves turning red or purple are all symptoms of strawberry pallidosis. Also, roots are brittle and show fewer rootlets.

Managing strawberry pallidosis

Unlike many other strawberry viral diseases, whiteflies bring pallidosis to the garden. Management strategies are the same for all strawberry viral diseases: only install certified disease-free plants, quarantine new plants, remove infected plants, and control whiteflies as much as possible.

Strawberry mottle is an unassuming viral disease that can cut your strawberry crop by 30%.
When strawberry mottle occurs alone, the damage tends to be relatively isolated. All too often, however, more than one virus appears simultaneously in a condition called virus decline. Virus decline can eliminate any chance of enjoying a sweet, juicy strawberry from your garden, no matter how well you care for your plants.

Vectors of strawberry mottle disease

Strawberry mottle is transmitted by insects, most commonly by strawberry, melon, and cotton aphids. Infected plants can also spread the disease. Unlike the strawberry mild yellow edge virus, which stays in an aphid’s gut for its lifetime, the strawberry mottle virus can only be transmitted for 2 or 3 hours after an aphid or other insect has fed on an infected plant, keeping outbreaks relatively localized. [Ten feet away probably looks impossible to a mostly flightless bug that is only 1/8” long.]

Symptoms of strawberry mottle

As insects pierce plant cells to suck out the sugary sap, viruses move from the insect’s saliva to the plant. As viruses tend to do, these pseudo-lifeforms start reprogramming plant cells to produce more viruses. All this reproduction clogs plant veins.

Strawberry mottle first appears on young leaves as smaller-than-normal leaves. They may also show yellow distorted areas. Stunting may occur, and they produce less fruit and runners than they might otherwise. As the disease progresses, symptoms become more severe, with older leaves turning red.

Strawberry mottle management

Strawberry mottle is more likely when plants stay in place over the winter, but that doesn’t mean you must rip out your plants each year. [Note: don’t rip plants out of the ground. Instead, cut them off at soil level to leave valuable soil microbes in place.]

To reduce the likelihood of strawberry mottle appearing in your garden, only buy certified disease-free plants and always place new plants in quarantine. As much as possible, try to control aphids around strawberry plants. If a plant becomes infected, remove it.

For some reason, strawberry plants often get infected with more than one virus simultaneously. Strawberry mild yellow edge virus is one of those diseases.

Strawberry mild yellow edge virus is a long name for a disease that can reduce your strawberry crop by as much as 30%. Strawberry mild yellow edge virus often appears when the mottle virus does. They are both transmitted by aphids. Nematodes may also add raspberry ringspot virus to the mix.

Strawberry mild yellow edge virus symptoms

As with most viral diseases, stunting is a common symptom of strawberry mild yellow edge virus. Older leaves may turn bright red, but the leaves around the crown nearly always exhibit yellow margins or edges, hence the name. These yellowed areas eventually die and turn brown. Leaf cupping may also occur.

Since these symptoms look much like water stress, fertilizer burn, overly acidic pH, boron toxicity, or bad weather, it is important to rule those out before deciding on a plan. Once strawberry mild yellow edge virus has made an appearance in your garden, there are steps you can take to minimize the damage.

How to manage strawberry mild yellow edge virus

Even though the fruits of infected plants are still edible, remove plants infected with strawberry mild yellow edge virus to prevent the disease from spreading. 

Aphids carrying the strawberry mild yellow edge virus are disease vectors for life. You can try to use insecticidal soap on every aphid that might be a carrier. Just be sure to do this at a time when honey bees and other pollinators will not be attending the flowers. Common lambsquarters and other Chenopods can also carry this disease, so keep these plants away from your strawberry plants.

This disease is most common when plants are grown using a matted-row method. The matted-row system allows parent plants to send out runners, or daughter plants, which will produce fruit the following spring. This highly productive method has been around for a long time. It gets its name because the runners end up intertwined, creating a mat. The only problem with the matted-row system is that it means plants are in place longer, making infection more likely.

As always, put new plants into quarantine until you know they are disease-free.