Bacterial head rot prevention and control

Cool winter and spring temperatures combined with prolonged periods of rain, fog, and dew provide the perfect medium for bacterial head rot pathogens. This means good air circulation between plants can go a long way toward preventing this disease. That’s a good thing, because chemical sprays and other treatments have not been consistently effective in preventing bacterial head rot.


The best way to prevent this problem in your garden is to start with resistant cultivars, such as broccoli with dome-shaped heads, space plants properly, and avoid overhead watering.

How to use a cloche

​Heat-loving plants, such as tomatoes, peppers, basil, and eggplant, get a much better start when protected with a cloche early in their development. Seeds or seedlings can be placed under a cloche after the soil has been watered. Most of the moisture will stay trapped in the cloche, but occasional waterings will still be needed as the plant grows. Because a glass cloche holds heat and air within, it can get too hot and too humid for your young plant. Occasional venting is necessary. To do this, simply slide a piece of wood or a rock under one edge of the cloche to increase air flow. In the evening, before temperatures start to drop, remove the prop and allow the cloche to sit flat on the ground again. If your cloche is made with fabric, venting is not needed. Once the plant outgrows the cloche, simply store it for another season or use it on another, smaller plant.


Cloches can also be used to help sensitive perennials make it through winter.

Junebug damage

Adult Junebugs feed on leaves. They fly in from weedy areas to feed at night. During the day, they tend to hunker down in a shady spot or burrow into the soil until dusk. The damage they cause is similar to Fuller rose beetles, earwigs, and snails. Grasshoppers and caterpillars may also cause similar damage. The only way to be sure is to catch them in the act.


The more insidious damage occurs underground as Junebug grubs feed on the roots of your lawn, especially ryegrass and bluegrass. Symptoms of infestation include brown, dying patches. If things get really bad, you can actually roll up patches of turf because it is no longer attached to the ground! Large numbers of Junebugs can defoliate a young tree in a matter of days. This is especially true for avocado trees, which may need to be protected with netting.

“Six years you shall sow your land and gather in its produce, but the seventh year you shall let it rest and lie fallow, that the poor of your people may eat; and what they leave, the beasts of the field may eat. In like manner you shall do with your vineyard and your olive grove.”
Exodus 23:11

The pulvinus causes movement by altering fluid pressure in the surrounding plant tissue. These changes in fluid pressure start when sucrose is moved from the phloem into the apoplast. The apoplast is the conjoined spaces between plant cells. As sugar is pumped into the apoplast, potassium ions are pushed out, followed by water molecules. This changes the pressure within the affected cells, causing movement. This is called turgor-mediated heliotropism. But not all heliotropic flowers have a pulvinus. Those that do not are still able to move by permanently expanding individual cells. This is called growth-mediated heliotropism. Pulvini are also used in response to nyctinastic and thigmonastic movements.


Heliotropic flowers

Heliotropic flowers face the sun from dawn to dusk. Slowly tracking the sun’s path across the sky, these flowers are believed to use heliotropism as a way to improve pollination, fertilization, and seed development. Heliotropic flowers often have five times as many beneficial insects present, due to the added warmth. [Many tropical flowers exhibit a modified form of heliotropism in which flowers maintain an indirect tracking of the the sun. This is believed to reduce the chance of potential overheating.] Beans, alfalfa, sunflowers, and many other species turn their blooms to follow the sun’s path across the sky each day. But sunflowers only use heliotropism in their early development, in the bud stage. Once a sunflower head emerges, it may track the sun for a short time, as an expression of phototropism, until the flower head reaches full size. The majority of sunflowers found in the northern hemisphere nearly always end up facing east.


Leaf heliotropism

Like floral heliotropism, leaf heliotropism is the method by which plants focus their leaves perpendicularly to the sun’s morning rays (diaheliotropism), or parallel to midday sun (paraheliotropism). Diaheliotropism allows leaves to capture the maximum amount of energy from the sun, while diaheliotropism protects plants from overheating and dehydrating.


How do your plants move during the day?

Periderm

The periderm is also made up of three layers: the cork, cork cambium, and phelloderm. Cork (phellem) is produced by a specialized layer of cambium tissue, known as the cork cambium, or phellogen. This cork cambium layer is only one cell thick and the cells divide in parallel (or periclinally) toward the outside of the tree. In some trees, the cork cambium layer also produces cells towards the inside of the tree. These inner cells are the phelloderm layer.


Function of cork

Cork keeps wine safe from the elements because it is impermeable to gases and water. Because of the cork, your wine stays where it is and (as long as the cork remains intact) will only grow better with time. The cork of a tree also blocks air and water. Cork is able to keep trees and wine safe from the elements, along with insects, bacteria, and fungal disease because it contains suberin. Surberin is a waxy material that creates a protective barrier. This barrier also blocks water and gas exchanges between the outermost layers of the tree killing the epidermis, cortex, and secondary phloem. This is the bark you see.


Trees and shrubs also use cork to cut off an unwanted body part (leaf, diseased twigs, mature fruit) from the rest of the plant. This is called abscission.

What is fruit?

Fruit is the fertilized ovary of a flowering plant (angiosperm). After pollination and fertilization occur, two new structures are produced: seeds (fertilized ovules) and pericarp (thickened ovary walls). In the case of apples and oranges, one flower produces one fruit. Sometimes, multiple flowers can fuse together to create a fruit. There are three different ways that this can happen:
​

• syngonium - a large, hollow receptacle holds multiple ovaries on the inside surface [figs]
• coenocarpium - the ovaries, floral parts, and receptacles of many flowers are clustered around a fleshy axis [pineapple]
• sorosis - ovaries of a group of budding flowers merge [mulberry]

In nearly every piece of literature you see, pineapples are listed as a common example of sorosis, but this is incorrect. I don’t know why they do this.


​How a sorosis fruit develops

If you look at a mulberry flower cluster, you will see several flower buds held tightly together. Each of these individual flowers open up, awaiting pollination. 

Berries vs. soroses

While mulberries may appear to have the same structure as blackberries and raspberries, botanically, they are quite different. Raspberries and other members of Rubus are made up of several drupes (a type of fruit) that are clustered around and attached to a dry thalamus. All of the drupes in a single fruit are made from a single flower. In mulberries, and other soroses, each rounded bit is its own fruit, formed from its own flower.

​Some garden words are fun to say. Schizocarp [ˈskitsōˌkärp] certainly qualifies.


A schizocarp is a type of dry fruit that splits into single-seeded parts, called mericarps, when ripe. Each mericarp is made from its own carpel. [A carpel is the female reproductive parts of a flower, including an ovary, stigma, and usually a style.] Mericarps can be dehiscent, which means they split open when ripe, or indehiscent, which means they stay closed.

The spaces between soil particles are called macropores and micropores. Macropores are greater than 0.08 mm and they hold air and water. Because the spaces are larger, water moves passively, pulled by gravity. Micropores are less than 0.08 mm and mostly hold air. Macropores are so small that the surface tension of water molecules means active suction must be used to pull the water out of these tiny spaces. Clay soil has far more micropores than macropores, so water and nutrients are held tightly, which means it is less porous.


Porosity, or permeability, refers to the ability of air and water to move through soil. Soil that is rich in organic material tends to have better porosity. Porous soil allows roots to find water and nutrients, and allows for healthy gas exchanges. Being flat, clay particles lie on top of each other like a deck of cards. This is why clay soil is so susceptible to compaction.


Clay and soil compaction

Compacted soil can create a barrier to roots seeking water, nutrients, and stability. It can even alter nitrogen, making it unavailable to plants (denitrification). This is especially true next to streets, driveways, buildings, and other heat islands. [If your unimproved clay soil ever feels spongey, it may indicate a masked chafer infestation.] On the other hand, leaching of nutrients is far less common in clay soil. More often, we end up with a super abundance of certain nutrients that creates an imbalance for our plants.


Clay soil and plant nutrients

Clay is made up of many negatively charged secondary minerals. That negative charge loves to attract and hold on to cations, or positively charged particles, such as potassium, zinc, and nitrogen. [That’s why most Bay Area soils have an abundance of potassium.] This attract-ability gives clay soil a high cation exchange capacity (CEC), which is a fancy ways of saying clay can hold onto 6 to 8 times more water and nutrients than sand. If you get a soil test (and I urge you to do so), you will also see a base saturation figure. To illustrate, CEC can be seen as the number of electrical outlets in your home, while base saturation is the number of those outlets currently being used. On average, sand has a CEC of 5-15, silt has a CEC of 8-30, and clay has a CEC of 25-50.


In the same volume of soil, clay has 100,000 times more surface area than sand, so there are plenty of places for attachments to occur. You can improve your clay soil’s base saturation by monitoring and correcting soil pH. Alkaline soils may need acidification, while acidic soils may need the addition of lime to bring the pH into a range suitable for plant growth (6.0 to 7.0). A proper pH can make important nutrients, such as iron, available to your plants.


Safety note: When planting trees around your home, keep in mind that root systems of plants growing in clay tend to be smaller, because so many more nutrients are available closer to the tree. This can result in a smaller in-ground support system for your tree, which makes it more likely to fall. Just sayin’…


Clay and drainage

When soil is extremely dry, it can’t absorb water because it becomes hydrophobic. Like a dry sponge, the water simply rolls off. Clay soil can act the same way. The rate at which water can enter soil is called its infiltration rate. Infiltration rates are given as millimeters of water absorbed per hour:

• sand - 30 mm/hr
• loam - 10 to 20 mm/hr
• clay - 1 to 5 mm/hr

How to prevent dieback

Plants that are healthy can often protect themselves from dieback. These tips can help reduce the risk of dieback in your garden and landscape:

• Ensure that plants receive enough irrigation in late fall and winter, to help them survive colder temperatures.
• Allow infected areas to go fallow.
• Use crop rotation to break the disease triangle.
• Prevent introducing these pathogens to your garden by quarantining new plants.
• Prevent water from traveling from infected plants to healthy plants.

Clearly, there are many causes of dieback. Taking the time to determine the reason for twigs and stems dying back can help you find an effective treatment.

Host plants

Nearly all fruit and nut trees, including cherries and kiwifruit, are susceptible to Phytophthora root and crown rot. But so are alfalfa, and members of the nightshade and cabbage families. This means tomatoes, eggplant, and potatoes are vulnerable, as are kale, cauliflower, Brussels sprouts, broccoli, kohlrabi, horseradish, cabbage, collards, turnips, rutabagas, radishes, bok choy, and mustard. And all because of too much water.
​

Symptoms of Phytophthora root and crown rot

Plants affected by Phytophthora root and crown rot look drought stressed. This is particularly unfortunate because the natural response is to provide more water - the last thing you want to do when Phytophthora is present. Symptoms normally start in just one branch or area of the tree or shrub before spreading to the rest of the plant. Leaves may turn purple or reddish. Sudden wilting and plant death may occur when the basal stem or crown are attacked, or the mold may attack the root system, causing plants to linger poorly for years before dying.

Preventing Phytophthora root and crown rot infestation

Proper water management is the best way to prevent and control Phytophthora root and crown rot. Never allow standing water to remain around tree and shrub trunks. Also, don’t let sprinklers hit tree trunks. These other tips can help you manage Phytophthora in your garden or landscape:

• Only install healthy-looking plants
• Do not plant plants too deeply or water excessively
• Allow soil to dry out between waterings
• Ensure proper drainage
• Quarantine all new plants for 4 to 6 weeks, including water that may drip from the containers
• Wash tools, boots, and hands after handling potentially contaminated plants
• Sanitize shoes with bathroom cleaner after walking through an infected area
• Do not add contaminated plants to the compost pile
• Use crop rotation to interrupt the disease triangle

You may be able to maintain an infected plant, with proper irrigation and good cultural practices, but it will never be the same. Phytophthora can stay in the soil for many years, so prevention is far easier than control.

The ginger plant

Ginger was one of the first spices to be exported from the Orient and it is a fascinating plant. As a plant family in its own right, ginger (Zingiber officinale) is cousin to turmeric and cardamom.


The ginger we eat is not actually a root. It is a rhizome. Rhizomes are modified, underground stems that put out lateral shoots and adventitious roots. Ginger plants do not have aboveground stems. Instead, they grow much like the grass in your lawn, with leaves rolled together at the base of the plant to form pseudostems, except that they can grow to three or four feet tall! Equally tall floral stems emerge directly from the rhizome. Flower buds start out green and then turn white and pink before opening up into mature flowers. Mature flowers can be pale yellow, deep purple, or brilliant red, depending on the variety.

Harvesting ginger

While you can harvest ginger rhizomes at any time, it is best for the plant’s long term health if you wait until the aboveground portion withers, similarly to garlic. The desired portion of the rhizome is cut off and the rest of the plant can be returned to its container. The cut off portion is then scalded to prevent it from sprouting. The older ginger gets, the tougher and drier the rhizome becomes. Ginger is a perennial plant, which means it keeps on growing. It may look as though it dies in winter, but don’t be fooled. Unless your region is too cold for ginger, it will come back year after year. Each little nub on a ginger rhizome is a potential new plant.

Pollination and fruit drop

Fruit drop can also be caused by insufficient pollination. This may be because a particular variety of fruit or nut tree needs a genetically compatible tree that it can use for cross-pollination. It can also mean there are not enough pollinators in your area. You can attract more pollinators to your garden by avoiding the use of broad spectrum pesticides and by installing a wide variety of flowering plants. Or, you can start raising honey bees, as I have! [Honey bees take up surprisingly little space and they boost pollination of nearly all your crops - plus, you get honey!]


Fruit drop and pruning

If you need to perform heavy pruning as fruit is developing, the tree may not have the food-producing capability that it had before the pruning job, so fruit drop will occur. Unless it is absolutely necessary, it is far better to leave pruning and tree training for the dormant season.


Fruit drop and the soil

Low magnesium (Mg) levels in the soil can cause fruit drop, as can high potassium (K) or boron (B) levels. You can’t know what your soil’s nutrient levels are without a soil test from a local, reputable lab. While they look convenient and appealing, over-the-counter soil tests are not yet good enough to be useful. I’m still waiting for some aspiring entrepreneur to make that one happen - there’s a huge market for it, but I digress. The type of soil can also have an impact on fruit drop. Sandy soils are far more prone to fruit drop than heavy clay soil.


So, don’t panic if your orange tree drops dozens or hundreds of tiny green fruits in May or June. This is normal. Just pick them up and add them to your compost pile. If you notice heavy insect infestations, signs of disease, chlorosis, or wilting, you will want to track down the cause and correct it for a healthy harvest later in the season.


And always remove fallen fruit and mummies, to avoid creating a disease triangle, or a hotel for pests.

Magnesium levels

​Too much magnesium in the soil makes it difficult for plants to absorb calcium and other anion nutrients, which can lead to blossom end rot, bronzing, and many other problems. This is a common problem in areas with alkaline soil. The opposite is true in areas with acidic soil. Insufficient magnesium symptoms look very much like potassium toxicity symptoms: older leaves, at the bottom of the plant, start turning brown, between and alongside the leaf veins, working upward through the plant. Magnesium deficiencies in stone fruits often start out as slightly brown areas along leaf edges (margins) that expand inward, causing cracking, necrosis, and leaf loss. Magnesium deficiency in California is extremely rare.


Stabilizing magnesium levels

Reaching and maintaining ideal mineral levels in soil for healthy plant growth is both science and art - mostly science. To start, get a soil test from a local, reputable lab. Unfortunately, over-the-counter soil tests are not yet accurate enough to be useful. Once you have your results, you can take these other factors into consideration:

• Local water supply Irrigation water that is more alkaline, such as we have in the Bay Area, tends to make calcium and magnesium less soluble, and harder for plants to absorb. This is because of carbonate and bicarbonate ions in the soil. The chemical transformation between these molecules also tends to leave salts behind.
• Soil pH Acidifying alkaline soil with ammonium sulfate or sulfur can help reduce the number of ions (Residual Sodium Carbonates) that make magnesium more available to your plants. If you have acidic soil, wood ashes can be a good source of magnesium and other plant nutrients.
• Soil structure Compacted soil makes it difficult for plant roots to access nutrients. Overly loose (sandy) soil cannot hold nutrients long enough for roots to find them. Incorporating organic matter as a top dressing will improve soil structure and soil health, leading to healthier plants.
• Compost Applying compost, mulch, and green manure are good ways to add magnesium and other plant nutrients to your soil.

Finally, schedule regular soil tests for your garden and landscape. Look at these tests as an annual physical for the living skin of your property. The information in these tests will help you make informed decisions about the magnesium in your soil.

If older leaves on cucumber, melon, or squash are turning yellow and leathery, the plants may be infected with cucurbit aphid-borne yellows.


This viral disease is transmitted by the cucurbit aphid-borne yellows luteovirus (CABYV). Luteoviruses are a genus of viruses that use plants as hosts, and are transmitted by aphids.

Symptoms of aphid borne yellow virus

Early symptoms are chlorotic (yellow) areas on lower leaves. These spots expand to include the entire leaf, leaving the larger veins bright green. The affected areas become leathery and brittle. Stunting and fruit drop are common as the plant struggles. Before genetic testing, this condition was attributed to plant aging (senescence), nutrient deficiencies, or other diseases, such as cucurbit yellow stunting disorder.


How the disease is spread

As the name suggests, this disease is spread by aphids. As aphids pierce plant tissue to feed on sap in the xylem, they infect the plants they eat. Once infected, the aphid will continue to spread the disease as it feeds. This disease can also be spread to lettuces, beets, and several weeds.


Controlling cucurbit aphid borne yellows

There is no way to control the virus, but you can reduce the presence of aphids in your garden with these tips:

• install silver reflective mulch at planting time to repel aphids (remove it as temperatures rise)
• install row covers to exclude aphids
• fertilize and irrigate plants properly, to keep them healthy
• avoid using broad-spectrum pesticides, which kill off beneficial predators that eat aphids
• clear aphid-bearing weeds from the immediate area

​
Infected plants should be removed and destroyed, to prevent the disease from spreading to nearby plants.

Fresh, sweet cherries are delicious, but cherry trees can be difficult to grow.


According to UC California Backyard Orchard, “cherries are the most difficult trees to keep alive.” If you are still determined, let’s see what we can learn about these trees.


People have been enjoying cherries since prehistoric times. Cherries are stone fruits, which means the fruit is a drupe. There are two types of cherry trees: sweet (Prunus avium) and sour (Prunus cerasus). The two cannot cross-pollinate with each other. Both types are native to Europe and western Asia. Sweet cherries are also known as wild cherries or gean.

Problems associated with neonics

The initial problems with neonics occurred when seeds were sprayed with the chemical and then put through a seed spreader that created clouds of neonicotinoids, killing tens of thousands of honey bees. Also, sprayed insecticides tend to go all over the place, causing overspray damage to nearby plants, waterways, and air. Of course, all this made the news and got everyone excited, but those particular problems have been resolved in most countries.


Now, neonics are more commonly applied as a drench, which is poured into the soil, to be absorbed by the roots. This eliminates overspray. Treated seeds are now managed in ways that prevent the pneumatic seed dispersal issue. Currently acceptable application rates seem to only be causing minimal harm to honey bee colonies, but they are still devastating to native bee populations.

Does your garden really need chemicals?

Individuals impact the amount of chemicals found in the environment by thinking before buying:

• Buy organically produced seeds and plants.
• Buy seeds and plants produced locally, rather than shipped across country, which requires pest-free plants, usually by heavy pesticide applications.
• Read the label. Just because a product claims to be “bee friendly” does not mean anything. Get to know the ingredients before you bring it home.
• Select plants suited to your microclimate, reducing the need for chemical interventions.
• Use integrated pest management (IPM) methods, rather than an ‘easy fix’ with long term complications.
• Tolerate a certain level of damage caused by insect pests.
• Maintain healthy soil. Healthy soil makes healthy plants that can defend themselves.

As we have learned in the past, spraying chemicals all over the place ends up causing unexpected problems. These chemicals start building up in our ground water and soil. Also, insects evolve much faster than we do. It is common for insects to develop a resistance to the poisons we spray on them, while we remain vulnerable.


Neonicotinoids may or may not be the next DDT. The truth is, we don’t know. What we do know is that there are better ways for home gardeners to care for their plants than to inundate the environment with chemicals.

How is latex different from sap, or resin?

Sap is the combined water, sugars, and plant nutrients that move through a plant’s vascular bundles to feed and water the plant. Resins, like latex, are protective substances that ooze from injury sites. Unlike latex, which coagulates and dries, resins create a hard, crystalline barrier.


How do plants make latex?

Latex is produced and transported in a separate system called the laticiferous system. There are two methods of latex formation and movement. Articulated laticifers consist of rows of plant cells found in the meristem tissue of stems and roots. The walls of these cells dissolve, creating tubes, called latex vessels. This method is common to poppies, fig trees, mulberries, rubber trees, and members of the sunflower family. Non-articulated laticifers, such as milkweed and spurge, develop a branching network of latex-producing cells throughout the plant. In some cases, the entire network is made from a single cell.


Plants that produce latex

There are over 20,000 species of plant that produce latex, occurring in over 40 plant families. Some of the more commonly known latex-producing families include:

• Asteraceae - artichokes, chamomile, chicory, dandelions, escarole, lettuce, sunflowers, oyster plants, tarragon
• Apocynaceae - [dogbane family, many are poisonous] vinca major, tiny periwinkle
• Asclepiadaceae - [milkweed family]
• Cannabaceae - hops, marijuana, hackberries
• Euphorbiaceae - [spurge family] poinsettias
• Moraceae - figs, mulberries, Osage-orange, banyan trees

Some mushroom, conifer, and fern species also produce latex as a defense mechanism.


Allergic reactions to latex
Because latex contains defensive chemicals, it can be an irritant. Prolonged exposure can lead to an allergic response, as can multiple surgeries, or spina bifida. Individuals with a latex allergy are at risk for anaphylactic shock and should avoid contact. Some forms of latex can cause blistering of the skin, or blindness, while other plants produce a latex with reduced amounts of the allergen. These forms are being researched as an alternative.


As you work in the garden, note which plants exude latex when damaged. And monitor your skin for reactions to this liquid plant defense.

Damage caused by citricola scale

Underneath those tiny domes of protection, citricola scale attach themselves to stems and leaves of citrus and pomegranate. They pierce the surface to reach the phloem, to siphon away valuable nutrients and sugary sap, weakening the tree. And they poop. This poop, called honeydew, contains a lot of sugar, and it creates the perfect growing medium for sooty mold fungus. Sooty mold blocks photosynthesis, further reducing your tree’s vigor. Citricola scale can reduce flowering and fruit production. During heavy infestations, twigs can be killed by citricola scale.


How to control citricola scale

Regularly monitoring citrus and pomegranate trees for these pests is your first line of defense. If you notice ant trails or sooty mold, take a closer look at twigs and leaves for signs of scale. Since ants protect these pests, you can eliminate that protection, making the citricola scale more vulnerable, by wrapping the tree’s trunk with a sticky barrier. Also, there are naturally occurring parasitic wasps that will control citricola scale insects (as long as you do not apply broad spectrum pesticides). Applying dormant oil in winter can also help reduce citricola scale populations.


Research has shown that 40% of citricola scale in San Joaquin Valley are resistant to organophosphates. It is believed that there is also a cross-resistance to malathion and carbaryl. This looks to be yet another example of chemical pesticides actually making the pests stronger, as we add more poisons to the environment and our food chain.


Bottom line, to control citricola scale on your pomegranate and citrus trees, inspect twigs very closely in April through June, and then look at the underside of leaves in late July. These pests can then be flicked off the leaf or stem with your fingernail.

Relatively new to the United States, the European pepper moth is poised to cause significant damage to gardens and commercial agriculture.


Each time an invasive plant or pest is brought into an area, there’s no telling what might happen. Resident predators or local diseases may make short work of the interloper. Then again, the insurgent may find a rich, predator-free environment perfectly suited to a population explosion. We don’t know, yet, which way things will go for the European pepper moth, but it’s probably a good idea to know what we’re up against, just in case.