Pseudomonas isn’t all bad

As handy as it would be to say that all Pseudomonas are bad, it ends up that some of these soil bacteria help plants stay healthy. In fact, they practically make life possible on Earth. Life sure is messy, isn’t it?


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 is also responsible for the formation of most of the 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.

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. If it is X disease, you will see pits and grooves near the graft union. If you cut into the wood, you will see brown areas in the phloem. These other symptoms 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 tasting.


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 they provide a source of infection to many important fruit and nut trees in our landscapes. Almonds, 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 they are non-reservoir hosts. This means that they can catch the disease but not spread it. Scientists don’t yet understand why that is, but they’re working on it.


X disease management

​Since scientists have been unable to reproduce the phytoplasmas responsible for X 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 responsible for spreading 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, it’s a good idea to get help. Contact your local Master Gardeners or Department of Agriculture for verification. They can also help you figure out the safe removal and disposal of any 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.

Preventing blackheart

To prevent blackheart from occurring, use these tips:

• Plant resistant varieties.
• Provide good drainage.
• Avoid overwatering and over-fertilizing.
• Do not harvest potatoes in extreme heat (above 86°F) or cold (below 35°F).
• Store harvested potatoes loosely in a cabinet, rather than tightly bagged or in the refrigerator.
• Store tomatoes in your pantry or on a countertop.

I hope the only blackhearts you have to deal with are the ones found in comic books.

Sometimes things look fine on the outside when the inside is damaged. This can happen to people. And it happens in beets, potatoes, seeds, and watermelons.


If you cut a beet, potato, seed, or watermelon in half and find the empty space in the center, it has hollow heart. Also known as hallowheart, this condition indicates that the plant faced more stress than it could handle. [The same might be said for people, but I digress.]

Sadly, this was the only licensable photo of hollow heart I could find.

Hollow heart of beets

If you cut into a beet and find an empty center, it’s time to get your soil tested. Hollow heart in beets is most commonly caused by a boron deficiency. If you look closely at the petioles, you will see that they are cracked, deformed, or smaller than normal. If the cavity is rotting, it may be black heart.


Hollow heart of potatoes

Hollow heart in potatoes is related to the water supply. And hollow heart is only one potato problem caused by irregular watering. Splitting and spraing (brown lines) are also water-related. Cold injury can also cause hollow heart in potatoes.

​Hollow heart in seeds

Garden peas and other seeds can also develop hollow heart. This occurs when unusually high temperatures occur right after germination begins. Because this disorder happens so early in the plants’ development, it leaves plants vulnerable to several fungal diseases. The best way to avoid hollow heart in peas and other seeds is to plant at the optimal time of year for each species.
 
Hallowheart of watermelons

Hollow heart in watermelons is mostly caused by insufficient pollen transfer. Poor pollination occurs when temperatures are low or when there are not enough pollinators in the area. Somehow, poor pollination causes the interior of a watermelon to develop triangular, often symmetrical gaps in the fruit. Seedless watermelons are more likely to develop hollow heart than seeded varieties, and orange and yellow-fleshed watermelons are the most likely to develop this condition. I have no idea why. What I do know is that you can attract more pollinators to your watermelon patch by adding flowers that appeal to both honey bees and bumblebees. You can offset low temperatures with row covers.
 

Hollow heart in watermelons can also occur when they are hit with too much fertilizer combined with not enough water, making the cells responsible for producing fruit unable to keep up with the cells producing rind.

​
Bottom line: a steady supply of water, planting at the appropriate time, and the right amount of nutrients can help prevent hollow heart from occurring in your garden.

Potato virus Y management

Chemical treatments are not effective against potato virus Y, so these good cultural practices are your best line of defense:

• Start with certified pest- and disease-free seeds and seedlings.
• Choose a resistant cultivar well-suited to your microclimate. [Belrus, Dark Red Norland, Eva, HiLite Russet, Kennebec, Monona, Norwis, and Sebago are all resistant varieties.]
• Space plants in such a way that minimizes exposed soil, or use reflective mulch. [The color difference between plant and soil has been shown to attract aphids.]
• Plant as early in the season as possible.
• Super light horticultural oils may be used to manage aphids. [Farmers also use systemic insecticides.]
• Remove and destroy infected plants.
• Start each growing season with new seed potatoes.

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. It’s all about 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 part of a potato plant’s defense mechanisms. They are produced in abundance when tubers are exposed to light. The green color under a potato’s 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. They get moved to a shipping truck and roll down the freeway with 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 of those potatoes will have shifted from storage mode to growth mode. That’s when trouble starts.


Is peeling enough?

Many people say that peeling the green skin away makes the spud safe to eat. That’s not entirely accurate. While most of those toxins are stored in the skin, they are still present in the rest of the potato. Just as a moldy cheese will also have mold growing throughout its interior, even if you can't see it. Peeling a green-skinned potato may not be enough. And cooking them does not affect the toxins.


All that being said, if you are generally healthy with a good digestive system, you may be fine with an occasional green-skinned potato. 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). What you can do is store your potatoes in a dry, cool 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 it was homegrown, you can.add it to the compost pile or plant it instead of putting your tummy at risk.

How to prevent powdery scab

The easiest way to prevent powdery scab in your potato patch is by planting only certified disease-free eye potatoes. I know it’s tempting to use spuds and other vegetable starts from the grocery store. They’re certainly convenient, and you already have them, 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 many years. If you are growing in containers, this is less of a problem. If a disease appears in that case, you can toss the soil in the trash and start over. Hopefully, you catch it before spores have been released. Other practices that will help prevent powdery scab include:

• Providing good drainage
• Letting the soil dry out between waterings
• Planting resistant cultivars
• Using the least amount of nitrogen needed

Infected potatoes aren’t the only way powdery scab can enter your garden. To prevent this disease, you need to monitor these other potential pathways:

• Bulbs, corms, rhizomes
• Firewood
• Planting containers
• Shoes and clothing
• Soil from introduced plants

I’m 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.

Symptoms appear more strongly in the second and third leaves, but are not likely in later growth. [Weird, right?] Unfortunately, the other symptoms include stunting and reduced fruit and runner production. Your strawberry crop can be reduced by nearly 20% because of vein banding. As soon as another virus takes hold, you can lose your crop entirely.


Strawberry vein banding vectors

Strawberry vein banding is generally carried by strawberry aphids. It can also be transmitted by  taking grafts from infected plants. Strangely enough, coming into contact with dodder can also spread the disease, but 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 be prevented by only installing certified disease-free plants, placing those plants in quarantine when they first arrive, and removing any plants that you suspect are infected. 


Since aphids can fly at certain 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 they should be avoided 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 strawberry plants.

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

Similar to other strawberry viral diseases, symptoms of strawberry pallidosis include stunting, significantly reduced fruit and runner production, and older leaves turning red or purple. An additional symptom of strawberry pallidosis is that roots are brittle and show fewer rootlets.


Managing strawberry pallidosis

Unlike many other strawberry viral diseases, pallidosis is spread by whiteflies. This makes controlling the disease more difficult. 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 you can.

Strawberry mottle is one of several viruses that can affect strawberries. Appearing on its own, the damage tends to be relatively isolated. All too often, however, more than one virus appears at the same time. Collectively, this condition is called virus decline and it can eliminate any chance at 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 carried by insects, most commonly by strawberry, melon, and cotton aphids. This virus is also spread by vegetative propagation of infected plants. Unlike the strawberry mild yellow edge virus, which is retained 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. This makes outbreaks remain relatively localized. [Ten feet away probably looks impossible to a 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 a plant’s cells to produce more viruses, which then clog the works. ​

Strawberry mottle first appears on young leaves as smaller than normal leaves that may also show yellow distorted areas. Plants may be stunted and they certainly 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 are left in place over the winter, but that doesn’t mean you have to rip out your plants every year. [Note: don’t actually rip plants out of the ground. Instead, cut them off at soil level to leave valuable soil microbes in place.]


As always, 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.

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 at the same time as the mottle virus, both of which are transmitted by some aphid species. 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 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 a lot like water-stress, fertilizer burn, overly acidic pH, boron toxicity, or bad weather, it is important to rule those things 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, plants infected with strawberry mild yellow edge virus should be removed to reduce the chance of spread. Aphids carrying the strawberry mild yellow edge virus are disease vectors for life. The only thing to do if the disease is present is to use insecticidal soap on each and 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 is a very productive method that 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 for a longer period of time. This makes infection more likely.


As always, place new plants into quarantine until you are sure that they are disease-free.

Strawberry crinkle virus symptoms

Wilting, reduced runner production, smaller fruit, deformed and/or streaked flower petals, and crinkled leaves are all symptoms of strawberry crinkle virus. Vein spotting may also be seen, as well as lesions on petioles (leaf stems) and stolons. Infected plants may appear top-heavy, exhibiting a form of epinasty. These symptoms can vary in intensity.


Strawberry crinkle virus management

Since bees are so important to strawberry formation, insecticides are generally not an option against the aphids that carry this disease. Use these tips to prevent strawberry crinkle virus from impacting your strawberry crop:

• only install certified disease-free plants
• place new plants in quarantine
• monitor for aphids and wipe or spray them off with a garden hose
• remove infected plants completely and immediately

Hopefully, your strawberry plants will never become infected with the crinkle virus. Until we figure out a sustainable way to get rid of aphids, well, be on the lookout.

No garden would be complete without the versatile, fast-growing zucchini. A favorite in stir-fry, breads, and the ever popular chocolate zucchini cake, zucchini can be very productive plant, as long as it stays healthy. [Email me if you'd like the recipe.]


Zucchini yellow mosaic symptoms

Whitened leaf veins, mottled, abnormally small leaves with alternating light and dark areas, and deformed, warty fruit are all signs of zucchini yellow mosaic. These are also symptoms of watermelon mosaic and papaya ringspot virus, two viral diseases that often occur at the same time as zucchini yellow mosaic. Watermelon mosaic infections tend to include blistered leaves, while zucchini yellow mosaic has the added symptom of leaf lobes becoming long and narrow, creating a ‘shoestring’ or ferny appearance.

Zucchini yellow mosaic host plants

In addition to infecting zucchini, zucchini yellow mosaic also infects other members of the cucurbit family, including melons, squash, pumpkins, some gourds, cucumbers, cantaloupe, and watermelon. The disease is transmitted by aphids.


Zucchini yellow mosaic management

As anyone who gardens knows, controlling aphids is difficult. These pests seem to appear overnight, in huge numbers. And all it takes is one aphid to get the whole process started. Unfortunately, insecticides are rarely useful in managing zucchini yellow mosaic, because the disease has often been transmitted before you even know the aphids are there. Reflective mulches can be used to discourage aphids, just be sure to remove the reflective material before it gets too hot. Row covers can also be used to reduce access to susceptible plants.


This disease can also be spread on infected garden tools and seeds, so be sure to sanitize your tools regularly and get your seeds from a reputable source (and not that zucchini from the grocery store).


Infected plants should be destroyed and replaced with resistant cultivars.


Since this virus is only viable for a few hours within their aphid carriers, creating a physical barrier of tall, non-host plants around your cucurbits can be enough to prevent the aphids from getting to the plants while the virus is still active.

Corn stunt does not mean ears of corn will suddenly start doing gymnastics over the fence into your neighbor’s yard. Instead, this bacterial disease will infect the phloem of corn plants, reducing them in size and all but eliminating kernel production.


Corn stunt disease complex

Some people see corn stunt as a single disease, while others see it as one part of a complex of three disease, the other two being maize bushy stunt mycoplasma and maize rayado fino virus (MRFV). Yet others include maize chlorotic dwarf virus in the corn stunt complex. Any combination of these diseases can be devastating to your corn crop.


Corn stunt symptoms

Healthy corn plants produce one or two ears of corn, depending on whether they are early or late maturing varieties, respectively. Plants infected with corn stunt are significantly shorter than normal, often only 5 feet tall, and may produce 6 or 7 ears. That may sound great, but it’s not. These ears are small and they do not fill properly, meaning there ends up being a lot of empty spaces. The kernels that do develop are not well attached, in a condition known as “loose tooth ears”. Infected plants will also exhibit pale yellow new leaves at the top. As these leaves mature, they tend to turn reddish.


How corn stunt spreads

Corn stunt is caused by Spiroplasma kunkeliiI, which is carried by leafhoppers. Corn leafhoppers (Dalbulus maidis), in particular, carry this disease with them, spreading it as they feed.


Corn stunt management

You can prevent corn stunt by using reflective mulches that deter leafhoppers. Planting your corn as early as possible in the growing season has been shown to reduce the impact of corn stunt infections. Apparently, the first generation of emerging bacterium are not as effective at spreading the disease as those that occur later in the season. Insecticides are generally not effective.

Juicy, sweet kernels of corn transform, overnight, into hideous, purple-grey, tumors. And these tumorous galls are delicious! 


Introducing, corn smut.

Now, corn has many pests and diseases: corn earworms, European corn borers, seed-corn maggots, soft rot, seed rot, fusarium root and ear rot, maize dwarf mosaic, pythium stalk rot, and damping-off disease, just to name a few. If you are an American corn farmer, corn smut is not what you want to see in your field. A lot of money and effort have gone into eradicating corn smut in North America. 


Corn smut in your garden is something else entirely.


While this distant cousin of mushrooms reduces crop size and makes ears of corn unmarketable for July picnics and canning purposes, it is edible and delicious. Unlike other corn problems, corn smut is said to taste like truffles, with a sweet, earthy, inky flavor. If it appears in your garden and you don’t want it, your local chef would love to hear from you!


To my way of thinking, if life gives you lemons, make lemonade. Or, if you are given corn smut, make quesadillas! Corn smut can be eaten raw, or added to many dishes, such as omelets, soups, sauces, meat dishes, or even desserts! As an extra bonus, corn smut is high in lysine. This means eating it with corn, or any other seed, provides a complete dietary protein.


Corn smut description
​
Also known as devil’s corn, common smut, boil smut, Mexican truffles, or huitlacoche [pronounced weet-la-COH-cheh], corn smut is a parasitic fungus that can occur on any aboveground portion of a corn plant as purplish blobs covered with papery greenish-white tissue. These fungi prefer meristem tissue and the galls are mostly seen on the ears of corn. Ear galls are significantly larger than those which form elsewhere on the corn plant.


Corn smut gets its purple color from pigments called anthocyanins. These are the same pigments found in blueberries, raspberries, and purple cauliflower. When you cook with corn smut, don’t be surprised to see the purple color change to black, because it will. Purple pigments generally don’t hold up well to heat.


Corn smut lifecycle

The corn smut fungus (Ustilago maydis) infects plant ovaries, causing kernels to swell up into large purple galls that are filled with fungal threads, called hyphae, and spores. Corn smut spores are already in the soil and can be carried on the slightest breeze or splashed water from rain or irrigation. Dry conditions and temperatures between 78°F and 93°F are all that corn smut needs to get started. Adding nitrogen or applying manure increases the chance of corn smut developing on your corn plants. Plant injuries also increase infection rates.


Corn plants try to defend themselves against corn smut by blasting the invaders with reactive oxygen (hydrogen peroxide). Sadly, from the corn plant’s perspective, this bubbling action simply spreads the smut spores.


If smut appears on your corn, fear not! Instead, harvest the galls while they are young and have the texture of a foamy popcorn, kind of firm and spongy. These moist galls are ready for harvesting 2 or 3 weeks after infection appears. As the galls mature, they turn dry and are mostly filled with unappetizing dry, black fungal spores.


Love it or hate it, corn smut is here to stay, so you may as well learn to cook with it (or sell it).

How to prevent watermelon mosaic

Spread predominantly by aphids and occasionally leaf miners, watermelon mosaic virus can also be carried on garden tools and clothing, so sanitize your tools regularly. The virus is only able to survive inside aphids for a few hours, so creating physical distance between potential carriers of the virus can also reduce infection. Crop rotation and removing infected plants can break this disease triangle.


Weeds, such as lambsquarters, cheeseweed, goosefoot, and Russian thistle, can act as vectors for this disease, so keep them away from your watermelon and other susceptible plants.


Horticultural oil spray can also interrupt transmission of this virus, but may cause problems of its own.


Insecticides are not effective because the disease is transmitted before the chemicals can kill the carrier. You can use reflective mulches under susceptible plants to repel aphids. If you use reflective mulch, be sure to remove it before the summer sun uses it to cook your plants.

We’ve all heard that beans cause gas, but did you know beans rust? Well, not rust like the undercarriage of a New England truck, but rust just the same.


Bean rust, like other plant rusts, is a fungal disease. Rust is found worldwide and it can wipe out your bean crop if it takes hold early enough in the growing season.

California’s cool, wet springs are just the conditions needed for rust to thrive. Add overhead watering or a decent breeze and the stage is set for an epidemic. Fungi are so efficient that, under ideal conditions, the disease cycle can be repeated every 10 to 14 days!


There are several strains of bean rust. Two of the most common are Uromyces appendiculatus and Uromyces phaseoli typica, but you don't need to know the Latin to recognize bean rust in the garden.

​
Bean rust symptoms

​Similar to other rusts, bean rust prefers moist places and moderate temperatures (65 to 85°F). While it can occur on any aboveground portion of a plant, bean rust is most commonly found on the underside of leaves. Pods can also be affected. At first, it just looks like tiny white or yellow bumps. Then those bumps break open and turn into bright orange, reddish, or yellowish flecks. Those flecks are pustules that are made up of more fungal spores than any of us cares to count. [Okay, some scientists love counting things like that.] A yellow outer ring is sometimes visible. Leaves may begin to curl downward and plants may develop a scorched appearance. These symptoms are easy to see and make identifying the condition simple. Getting rid of it is something else all together.


Bean rust control

The fungi that cause bean rust can be spread by ants, aphids, and gardeners. It can stick to tools, fingers, and clothing. As with many other plant diseases, prevention is far easier than eradication. Use these tips to prevent and control bean rust in your garden:

• Space plants to allow for good air circulation.
• Avoid overhead watering.
• Plant resistant varieties.
• Monitor plants frequently.
• Apply sulfur to make leaves and pods less hospitable to bean rust.
• Remove infected leaves and dispose of them in the trash.
• Avoid excessive use of nitrogen and overfertilizing. 
• Cut spent (uninfected) plants off at soil level and add them to the compost pile.
• Use a 2-year crop rotation with bean plants.
• Infected leaves should be removed and thrown in the trash.

Keep in mind that rust pustules are easily dislodged and can land somewhere else, or on the soil, where they can be bounced back up into your plants by rain, wind, and overly exuberant irrigation. And be sure to disinfect your tools after removing rust-infected leaves, to avoid spreading the fungus to healthy plants.

The halo blight bacteria (Pseudomonas syringae pv. phaseolicola) enters plants through tiny wounds, often caused by insect or herbivore feeding or injury, and through natural openings, such as the stoma.


Symptoms of halo blight

Halo blight is frequently confused with bacterial brown spot and common blight. In all three cases, small, water-soaked lesions appear on leaves.


​In common blight, those lesions have wide, lemon-colored borders, and they continue to grow. Bacterial brown spot lesions have narrow light green borders and the centers tend to dry out and look tattered. Halo blight lesions tend to stay small and they have prominent light green halos, hence the name.


Leaves are not the only place damage occurs. Pods can also become infected, making them inedible. Pods infected with common blight have lesions with red or rust colored borders, while symptoms of the other two diseases are difficult to distinguish from each other, both being the same water-soaked lesions seen on leaves.


Managing halo blight

As with other diseases, prevention is the easier way to go. Since moisture is needed for halo blight to develop and spread, avoid overhead watering and save the sprinklers for your lawn. Furrow irrigation will get water to the roots without creating a potential disease site. This is especially important when temperatures are between 68 and 74°F (20 to 23 °C). Unlike many other blights, halo blight bacteria prefer these slightly cooler temperatures.


Also, be sure to start with certified disease-free seed, and place new plants into quarantine before exposing the rest of your garden to whatever they may be carrying. Speaking of carrying disease, the bacteria responsible for halo blight can also travel on rain splashes, wind, pet fur, shoes and clothing. If you have been exposed to halo blight, you might want to change your clothing and swap shoes to avoid spreading the disease throughout your garden. Finally, when your bean plants have completed their life cycle, cut them off at ground level, leaving soil microorganisms in place, and add plant debris to the compost pile. Leaving plants to break down in the garden can provide potential overwintering sites.


Fixed copper or Bordeaux mixture may be used to prevent halo blight. Plants infected with halo blight should be removed and destroyed, followed by a 2 to 4-year crop rotation program.​