​You can find lists of viral diseases common to other areas by contacting your local County Extension Office.


Controlling viral diseases in the garden

Healthy plants are better able to ward of viral infections. This means proper feeding, irrigation, and pruning. It also means selecting resistant plants that are suitable to your microclimate, buying only certified disease-free plants and seeds, planting at the proper depth, and avoiding mechanical injuries from rubbing branches and weed wackers, among other things.


Use an integrated pest management (IPM) program to control viral diseases. This means encouraging natural predators and parasites of viral diseases, sanitizing garden tools regularly, and using cover crops and crop rotation to interrupt disease triangles. Yellow sticky sheets can be used to trap many disease-carrying pests. Pesticides and insecticides used to kill disease carriers are not effective. Reflective mulches have been used successfully to confuse some disease-carrying pests. Diseased plants should be removed and thrown in the trash to prevent healthy plants from becoming infected.


Viral diseases of plants are on the rise, largely due to monoculture, mass production, climate change, global shipping and other human activities. You can reduce the likelihood of viral diseases affecting your plants by placing new plants in quarantine and knowing what to look for.


Now you know.

Brown soft scale lifecycle

Brown soft scale females give live birth, or lay eggs which hatch almost immediately. These young crawlers move around freely on leaves and twigs, feeding as they go. They continue moving around until they are about half grown, molting twice.


Controlling brown soft scale

Sticky barriers around the trunks of susceptible trees can cut off protection to brown soft scale pests by ants. Also, avoiding the use of broad spectrum pesticides allows natural predators to feed on these pests. If you see scale shells with holes in the top of them, you will know they have been parasitized by beneficial insects. Heavy infestations can be treated with dormant oil in winter, but this is rarely necessary.

The invasive Asian citrus psyllid carries huanglongbing, a deadly citrus disease. Orange, lemon, lime, kumquat, and grapefruit trees infected with huanglongbing must be destroyed by a professional. Sad, and expensive. These pests, when present, are most active April through June in the San Francisco Bay Area.


Psyllid species most likely to threaten your garden include:

• Asian citrus psyllid (Diaphorina citri)
• Eugenia psyllid (Trioza eugeniae)
• olive psyllid (Euphyllura olivina)
• pear psyllid (Psylla pyricola)
• potato/tomato psyllid (Bactericera cockerelli)

There are dozens of psyllid species that infest ornamental trees and shrubs, as well. These include the recent invasions of Ficus leaf-rolling psyllids and spotted gum psyllids. On the other hand, some psyllid species are being used to our advantage. The Australian melaleuca psyllid, for example, has been purposefully introduced to Florida to help control paperbark trees, an invasive weed tree.

Damage caused by psyllids

One of the biggest problems associated with psyllids is their poop. After they have robbed your plants of valuable nutrients, weakening the plant, they add insult to injury by excreting a large portion of the sap they stole and depositing on leaves. Known as honeydew, the excrement of sap-suckers is filled with sugar and other nutrients. Honeydew ends up being food for fungal sooty mold and disease-carrying ants. ​

When disturbed, leafroller caterpillars tend to wriggle wildly and then rappel to the ground on a single silken thread.

Eggs are very tiny, only 0.04” in diameter, flattened, oval, and white. Larvae grow to 0.75” in length and are white, with an amber colored head. Larvae are flattened, with a widened area just behind the head, tapering towards the rear end. Pupae are also whitish and flattened, getting darker as they mature.


Pacific flathead borer lifecycle

These pests overwinter in a prepupal stage. As temperatures begin to rise, they pupate. From April through July, adult beetles emerge, usually beginning around the same time apple trees are blooming. Then females mate and begin laying eggs in the bark, favoring areas weakened by sunburn or mechanical injury from tree supports, weedwackers, and out of control lawn mowers. When the eggs hatch, larvae burrow directly into the bark and begin feeding on the nutrient-rich cambium layer, robbing your trees of the nutrients they need to grow and produce fruit. As the larvae mature, they will either create a pupal chamber in the xylem, or burrow under the bark, where they will stay until the following spring. ​

Pacific flathead borer controls

Healthy trees are better able to resist and recover from Pacific flathead borer attack. This means selecting trees suitable to your microclimate, planting them at the proper depth, and feeding, irrigating, training, and pruning them properly.


Since eggs are laid in weakened bark, protect trees from mechanical injury, and be sure to whitewash exposed bark before sun damage can occur.


Birds, especially woodpeckers, will find and remove Pacific flathead borers, and carpenter ants eat both larvae and pupae. Insecticides are commonly used in commercial orchards to kill new larvae, but once the larvae are inside the tree, there is nothing you can do besides pruning out infested wood and burning it.


Now you know.

Preventing ashy stem blight

Ashy stem blight is known as a “stress pathogen”. This means it preys on stressed plants. Stresses, such as a heavy fruit load, high temperatures, drought, and water-stress can make plants more susceptible to infection. Keeping your plants healthy can help them protect themselves.


While furrow-irrigated plants rarely have severe cases of ashy stem blight, you may be surprised to learn that the disease is common with drip-irrigated systems. It is believed that this particular set-up increases salt levels near the soil surface, creating salt stress.


Monitor plants regularly for signs of infection. Once infection occurs, affected plants should be removed and thrown in the trash, and a 3-year crop rotation with non-susceptible crops should be put into place. There are no effective chemical treatments for this disease.


Now you know.

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.

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 fo 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.

You don’t have to grow tobacco to have reason to worry about tobacco mosaic virus. 


Tomatoes are highly susceptible to this disease that can be carried on tools, clothing, cigarettes, and, yes, even the saliva and other bodily excretions of smokers. This tenacious virus can stay alive even after its host is dead, and it can withstand extreme temperatures. 

How is tobacco mosaic virus spread?

Unlike many other diseases, which are spread by sap-sucking insects, such as thrips and aphids, tobacco mosaic virus is mostly spread by direct contact. Tobacco mosaic virus has also been spread by chewing insects, such as grasshoppers and caterpillars, and by bumblebees, as they pollinate flowers.


Preventing tobacco mosaic virus

Plants infected with tomato mosaic virus must be removed and destroyed. According to the  Michigan State University Extension, you can prevent the virus from moving onto uninfected plants by spraying them, just before transplanting, with a 20% nonfat dry milk solution. This spray can also be used on containers, walkways and other surfaces. The milk solution coats the virus, rendering it inactive. The milk treatment only works while the milk is wet.


These other tips can also help reduce the likelihood tobacco mosaic virus in your garden:

• Quarantine all new plants until you are certain they are pest and disease-free.
• Buy disease resistant varieties, especially when shopping for heirlooms and hybrids.*
• Wash your hands before working in the garden.
• Wear garden shoes, and not street shoes, when working in the garden.
• Avoid taking cuttings from potentially infected plants.
• Crop rotation can help break the disease triangle, though this virus is tough.

*Check plant labels for the letters V, F, N, T or A. These symbols indicate a resistant to verticillium wilt, fusarium wilt, nematodes, tobacco mosaic virus or alternaria stem canker, respectively.


According to a study conducted by the National Institutes of Health, all brands of cigarettes studied tested positive for tobacco mosaic virus, while only 53% of those viruses were viable. Also, 45% of the saliva samples taken from smokers tested positive for tobacco mosaic virus. So, smokers and users of other tobacco products, please wash your hands before entering your (or someone else’s) garden, and always throw your butts in the trash. Thank you.

Preventing the spread of tomato yellow leaf curl

Tomato yellow leaf curl has the potential to temporarily eliminate tomato growing in certain areas, including your garden. This disease is the reason why tomatoes are generally not grown in tropical and semi-tropical regions. Cold winters tend to kill of the whitefly vectors, but the San Francisco Bay’s winters are not necessarily cold enough to provide that protection. Instead, you can use these tips to protect your tomato plants, and tomatoes, in general:

• quarantine all new plants to ensure they are disease and whitefly-free
• select varieties that are resistant to the tomato yellow leaf curl virus
• avoid transplants from affected areas (Mexico, Florida, Georgia, and Texas)
• manage whiteflies
• use crop rotation
• cover tomato plants with floating row covers
• remove weeds from the area
• remove and destroy crop residue, rather than leaving it in place

​Now you know.

    Prunus stem pitting causes later then normal leafing out. Leaves look pale and tend to wilt in summer, turning red or purple earlier in the season than is normal. Fruit size, quantity, and quality are significantly reduced. All of this is because the virus interferes with the flow of water and nutrients through the graft union, effectively starving the tree. These symptoms look very much like root damage caused by rats and voles, girdling roots, and fungal diseases of roots. The difference being that the tomato ringspot disease causes the bark, both above and below the soil line, to thicken and become spongy. This weakened area often allows the tree to topple over. Before that happens, you will also see pitting in the sapwood of the trunk. Usually the tree dies before pitting is seen in any branches.


How to control tomato ringspot

In a word - you can’t. The disease is incurable and infected plants put nearby plants at risk. The only thing you can do is remove the infected plants, and those nearby, and burn them or toss them in the trash. Just because symptoms disappear does not mean the infection is gone. Plants that no longer show symptoms are still carrying the disease, which can then spread throughout the garden. Once plants are removed, the affected area should be allowed to go fallow for at least 8 months, to starve out any dagger nematodes that may be lurking underground, and remove any potential disease-carrying weeds.


To prevent tomato ringspot, there are a few steps you can take:

• quarantine new plants
• use only disease-free plants
• plant disease-resistant rootstocks
• disinfect tools regularly
• remove infected plants and those surrounding them as soon as the disease is seen

Yes, removing plants from the garden or landscape is disappointing, but having to remove even more plants because of an initial delay could be both costly and heartbreaking. 


And you thought this post was going to be all about tomatoes…


So did I!

Avocados and exudates

Peach, citrus, and pecan are just a few of the more than 200 tree species impacted by this problem, as are grapevines, but the sugar volcano is specific to avocados and box elders.


Exudates are secretions. When an avocado tree is first attacked by shot hole borers and Fusarium dieback, it will respond by pushing a sugary exudate out of the borers’ entry and exit holes. This sugary secretion often includes frass and sawdust. As it dries, this sugary exudate forms a white, crusty ring or cone-shaped patch on the affected branch.


And these crusty white patches, dear readers, are sugar volcanoes.


Now you know.

Plum bud gall mite identification

In late winter, galls begin to form around these tiny invaders. By spring, adults emerge from their protective galls. At 1/100th of an inch in length, these mites are too tiny to see with the naked eye. If you have a 20x hand lens, you may be able to see them, if you look very closely. They can be a translucent yellow, pink, white, or purple, with two pairs of legs up near the head. You are more likely to see galls on new shoots and fruit spurs that plants produce in response to these invaders. Galls are warty, bumpy growths that don’t look like normal tissue. ​

Controlling plum bud gall mites

Treating your trees with wettable sulfur in March or April, when plum bud gall mites first start to emerge from their protective galls, has been effective in controlling these pests in other regions. Treatments may need to be repeated, depending on the level of infestation. Note that apricot leaves are very sensitive to sulfur, so you can only treat apricot trees with sulfur before leaves emerge. Because these particular eriophyid mites are new to the region, we do not yet know what sort of an impact native predatory insects will have on controlling plum bud gall mite populations.


If you happen to see this new pest on your trees, please contact your County Extension Office right away.

How to control beet armyworms

In the home garden, natural predators are your plants’ best defense against beet armyworms. Predatory wasps will parasitize beet armyworm larvae, while big-eyed bugs, and minute pirate bugs will feed on the eggs. Spiders, damsel bugs, assassin bugs, tachinid flies, and lacewings will also feed on beet armyworms, so avoid using broad spectrum pesticides. In severe cases, you can apply spinosad or a specific type of Bacillus thuringiensis (ssp. aizawai).


Prevent beet armyworm invasions by monitoring nearby weeds, especially lambsquarters, goosefoot, and pigweeds for signs of egg clusters.


Harvesting your crops as soon as they are ready can also interrupt the lifecycle of these pests.


Beet armyworms have been known to travel as far as 10 feet during a night, putting most of your garden plants at risk. Monitoring for signs of beet armyworm infestation can help you prevent the problem from spreading.

Unless you want to apply chemical fungicides, you are best off selecting varieties that are resistant to this disease in the first place. In the world of almond trees, the following species are most susceptible to blossom brown rot: Butte, Carmel, Drake, Ne Plus Ultra, Winters, and Wood Colony.


You can also reduce the likelihood of blossom brown rot by removing all mummies, as soon as they are seen, and disposing of them in the garbage. Pruning and training for better air flow can also reduce the amount of time blossoms take to dry.

Curly dwarf may sound like the punchline from a bad joke, but this viral disease can ruin your artichoke plants.


Curly dwarf is spread by insects, and can be fatal, so knowing what it looks like can help you keep it from spreading to uninfected plants. While only found on artichokes, in the field, cardoons, sunflowers, and zinnias have been infected in laboratory tests.


[Unfortunately, I was unable to find a single image of an artichoke plant infected with curly dwarf, but I will keep looking. Please let us know if you have one!]


Curly dwarf, also known as artichoke curly dwarf, is caused by the artichoke curly dwarf virus (ACDV). While very little is currently known about this particular virus, we do know that it is almost found in tandem with another virus (Artichoke latent virus), which seems to have no disease symptoms.


Symptoms of curly dwarf

Severe stunting, leaf curling, and reduced bud production, with buds remaining small and often misshapen, is a clear indication that your plant has become infected with curly dwarf. Leaves may also have dark, dead areas.


Preventing curly dwarf

We do not yet know which insects spread curly dwarf, but we do know that it can be transmitted to uninfected plants. For this reason, it is important to remove any infected plants as soon as they are identified. The curly dwarf virus is commonly spread when infected plants are divided for propagation purposes, so only use certified disease-free plants.


Since the virus also lives on milk thistle (Silybum marianum), keeping those weeds away from your artichoke plant may reduce the chance of infection.

Whereas other mites produce webbing and cause leaf drop, the European red mite produces little or no webbing and no leaf drop.


Persistent, heavy mite feeding can also cause transpiration burn (leaf blackening), reduced fruit size and quality, shoot growth, trunk and limb growth, and root growth.


If populations of European red mites become significant, you can apply delayed dormant horticultural oil, but that oil may cause sunburn damage. It’s a tough call. Since European red mites have demonstrated resistance to miticides (a type of pesticide geared toward mites), it is better to avoid chemical sprays. Spraying these pests with a hose does nothing.


If your garden or landscape has a lot of biodiversity, odds are pretty good that there will be enough predators to control European red mite populations. Also, keeping plants dust-free makes the environment less hospitable to these pests.