- Author: Help Desk Team
As tomato growers, we might consider any ailing tomato to have a disease. That would end up making this blog post really, really long. However, we need to consider diseases separately from damage done by pests (both vertebrate and invertebrate—see blog post from July 18: Tomato Diseases in the Home Garden) or environmental disorders such as blossom end rot (see blog post from July 4: https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=57297). We will cover fungal, viral, and bacterial diseases here.
Fungal
The most common tomato diseases are fungal. Many of these fungal spores are nearly everywhere all the time, and given the right conditions, will find a home on your tomato plants. Some fungi prefer cool conditions while some prefer warm. Most prefer wetness and high humidity. Crowding can prevent proper air circulation and encourage fungal diseases. Nutritional deficiencies and injuries also encourage fungal diseases.
Damping Off Disease typically affects seedlings. It's caused by various soil fungi that grow under damp conditions. The affected tap roots of seedlings in contaminated, overly damp soil are dark and mushy and the seedlings usually die. Prevent this by using fresh clean soil and sanitized containers with good drainage. Use alcohol or a 10% bleach solution to sanitize. https://ipm.ucanr.edu/PMG/PESTNOTES/pn74132.html
Septoria Leaf Spot is favored when plants are exposed to cool, rainy weather or splashing from soil. It's relatively uncommon in Contra Costa County. Prune off infected leaves and twigs and keep plants dry. https://ipm.ucanr.edu/PMG/GARDEN/PLANTS/DISEASES/septorialfspot.html
Phytophthora Root and Crown Rot is a soil-borne fungus-like organism transmitted under wet conditions by splashing water or contaminated garden debris, pots, or tools. Plants appear drought-stressed since the vascular system is compromised, and often die. It can also cause damping-off. Look for darkening of the crown, roots, and stems. Good drainage, avoiding overwatering, and sanitizing any tools used on the diseased plants are preventive measures. In previously affected soil, avoid planting members of the Solanaceae family (tomatoes, eggplant, peppers, potatoes) in the infected soil and plant a resistant crop instead such as corn. https://ipm.ucanr.edu/PMG/PESTNOTES/pn74133.html
White Mold, also called Cottony Soft Rot, appears as bleached areas on stems from white mycelia. Fruit can be affected and appears gray. The mold favors hot, moist conditions and often spreads from dying flowers. Bury or dispose of infected tissue; avoid overhead watering, overwatering, and crowding. https://ipm.ucanr.edu/PMG/GARDEN/VEGES/DISEASES/tomwhmold.html
Powdery Mildew looks different on artichoke, pepper, and tomato leaves from its appearance on other plants (see reference). It requires warm weather and living tissue to grow but does not require moist conditions. Prevention includes planting resistant varieties in sunny areas and avoiding crowding. Occasionally a fungicide or a biologic is needed. https://ipm.ucanr.edu/PMG/PESTNOTES/pn7406.html
Early Blight's name is somewhat misleading as it occurs on mature tomato plants and can affect fruit. It is uncommon in our Mediterranean climate. See reference for photos, prevention, and treatment: https://ipm.ucanr.edu/PMG/GARDEN/VEGES/DISEASES/tomearlyblight.html
Late Blight (Phytopthora infestans) occurs in our coastal areas and is favored by average temperatures and high humidity. It can spread rapidly from other Solanaceae family members or their cullings. Avoid overhead sprinkling and crowding of plants and buy certified blight-free seeds and tubers. Dispose of affected plants and debris in green waste. https://ipm.ucanr.edu/PMG/GARDEN/VEGES/DISEASES/lateblight.html
Black Mold typically affects ripe tomato fruit during conditions of warmth and high humidity. It can appear as small dark brown spots or grow into large, sunken areas. Pick fruit as soon as it ripens. https://ipm.ucanr.edu/PMG/GARDEN/VEGES/DISEASES/tomblkmold.html
Verticillium and fusarium wilt are fungal diseases that will be discussed in an upcoming blog.
Viral
Tobacco Mosaic Virus is a disease primarily of Solanaceae, of which tobacco is a member. It is transmitted by infected seeds or by tobacco residue on the hands of smokers. The leaves appear mottled and stringy, but the fruit is edible. Herbicide damage can appear similar. Many tomato cultivars have resistance. https://ipm.ucanr.edu/PMG/GARDEN/VEGES/DISEASES/tobaccomosvir.html
Curly Top Virus and Spotted Wilt Virus are fairly unusual afflictions. Curly Top Virus is carried by the beet leafhopper and causes curling, puckering, and stunting of leaves. Fruit is usually discolored and small. Spotted Wilt Virus is transmitted by the western flower thrip. It can be difficult to diagnose as it presents differently depending on the stage of plant growth. Fortunately, it's rare in Contra Costa County. Both have wide host ranges. For severe infections in the garden, testing may be warranted since the viruses can mimic other diseases. Affected plants will need to be removed and disposed of. Unfortunately, insecticides do not kill thrips or leafhoppers in time once the damage has been noticed. https://ipm.ucanr.edu/PMG/GARDEN/VEGES/DISEASES/curlytop.html
https://ipm.ucanr.edu/PMG/GARDEN/VEGES/DISEASES/tomspotwltvir.html
Bacterial
Bacterial Speck, Bacterial Spot, and Bacterial Canker are all easily confused. They tend to be introduced on infected seeds, and can overwinter on garden debris, flats, and stakes. They all prefer wet conditions. All three cause lesions on fruit. Speck and Spot cause similar leaf lesions and are chiefly distinguished by their appearance on fruit.
Bacterial Speck prefers cooler conditions such as in coastal regions. A Pseudomonas bacterium causes small sunken spots with white halos that can become scabby. The leaf spots are similar and appear greasy, and leaf margins can turn brown in an angular pattern. Stems can also be affected.
Bacterial Spot is caused by Xanthomonas bacteria, resulting in large, black, sunken spots on fruit and irregular black spots on leaves. Warm, humid conditions favor its appearance. The fruit may be eaten once the black spots and any underlying maceration are removed. https://u.osu.edu/vegetablediseasefacts/tomato-diseases/bacterial-leaf-spot/basics/
Bacterial Canker also prefers warm, humid conditions. Stunting, wilting, scorching of leaf margins, cankers on stems, and vascular discoloration are distinguishing features.
https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=27926&postnum=27926
Bacterial diseases can be managed somewhat with copper sprays and these sprays are acceptable for organic farming.
Prevention
Prevention is the ideal way to manage all these diseases:
• Rotate crops
• Don't overwater or spray the plant
• Don't crowd plants
• Remove lower leaves that can touch the soil
• Use mulch to prevent spread from soil
• Buy resistant varieties
• Control weeds to prevent thrip and leafhopper invasions
• Dispose of infected plant parts and debris, sanitize tools, planting flats, and hands
• Baby heirloom tomatoes because they typically have little resistance
For more information about pests and diseases of tomatoes, see this website: https://ipm.ucanr.edu/home-and-landscape/tomato/index.html
Help Desk of the UC Master Gardeners of Contra Costa County (EAS)
- Author: Ben Faber
So a question comes in about a problem with a backyard avocado tree. And it would seem the first question would be about the overgrowth happening at the base of the trunk. This a ‘Fuerte' avocado that is grafted on a seedling avocado rootstock. It's not unusual to see an overgrowth, but this is the most extreme example I have ever seen. So it's basically an incompatibility between the graft and the rootstock. In many cases this is no big problem and trees can live a long time, as this tree has.
But the homeowner wasn't asking about the unusual growth at the base, but the canker that had appeared in the center of the trunk near the base.
This has the classic white sugar exudate that occurs with a wound of any kind in avocado. The sugary sap that contains the unusual mannoheptulose 7-carbon sugar characteristic of the laurel family to which avocado belongs will ooze out of the wound and result in a white crust (Read more about this sugar at: https://www.sciencedirect.com/science/article/pii/S0254629911001372 ).
Anyway, so this backyard tree is in an area that is getting 10 minutes of lawn watering a day. Lawns and avocados don't get along. And avocados don't get along with short, shallow irrigation that result in salt accumulating in the root zone. Which is what has happened here. Salt stress and the result is an infection of bacterial canker (https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=7920 ).
It's not fatal in an old tree like this, but it can predispose the tree to root rot. And that's not something that is easy to treat in backyard settings.
- Author: Ben Faber
A recent grower survey in Santa Barbara County asked a whole bunch of questions. One of which was had they had an evaluation of irrigation distribution uniformity. This is a free service that can significantly improve on-farm water use and most importantly improve plant health. Avocados that don't get the right amount of water at the right time are extremely susceptible to root rot. Proper irrigation is the first line of defense against root rot, good farming that results in good economic returns to the grower.
So, with a free DU available to growers, how many do you think took advantage of the service? Barely 50%!!!!!!!! This just does not make sense. In a land of little water and frequent examples of what can happen with no water ………………..and high priced water, what is going on?
The local Resource Conservation District has done many system evaluations, and most results find that improvements can be made in distribution uniformity. This is true in relatively new irrigation installations. It does not take long for problems to occur in even well designed and installed systems.
During the summer of 2007, the Casitas Municipal Water District (CMWD) contracted with the Irrigation Training and Research Center (ITRC) of California Polytechnic State University, San Luis Obispo, to conduct field evaluations of drip/micro systems. A team of two students conducted 35 field evaluations.
Distribution Uniformity (DU) – DU is a measure of the uniformity of water application to trees throughout an orchard, with DU = 1.0 being perfect. The measured orchard DUs in the Santa Barbara/Ventura area had an average DU of 0.66, while the California state average for drip/micro is 0.85.
In general, there were substantial opportunities to improve the distribution uniformity (DU) of the water to trees throughout an orchard. An improved DU will minimize over-irrigation in some areas, and reduce under-irrigation in others. Key recommendations that were provided included:
Install a pressure regulator at the head of every hose
With a regular microsprinkler, doubling the pressure causes about 40 percent more water to come out of the nozzle. Pressure regulators are added to have similar pressures throughout the orchard and thus reduce the risk of over-irrigating portions of the field. On many farms, the difference between the highest pressures was double or even triple the lowest pressures (40-70% more water). By adding the correct high-quality, pre-set pressure regulators with the correct flow rate rating, the farmer can get similar pressures to every nozzle and prevent over-irrigation.
For a pressure regulator (PR) to work, more pressure must enter the PR than what the PR is rated for. For example, to use a 25 psi PR, you need at least 27 psi into the PR. All a PR does is reduce pressure; it cannot add pressure.
Another problem on hillsides is that some pipes have as much as 100 psi before the PR. A PR can effectively reduce the pressure down to 50%. What is recommended in these fields is to reduce the pressure in the pipe by adding an in-line valve halfway down the hill and throttling it down to a reasonable pressure.
Completely replace all microsprinkers with pressure compensating microsprinklers
Pressure compensating microsprinklers have an internal flexible diaphragm that reduces a pathway as the pressure increases. These allow similar amounts of water to get the trees even if the hoses do not have the same pressures. Whenever the pressure is doubled, 10 percent more water will come out of these emitters, compared to 40 percent more water with a regular microsprinkler. Having pressure compensating emitters can drastically improve the DU in virtually every avocado orchard because most irrigation systems were not properly designed for microsprinkler systems, or because the farmer has altered the original design by adding different-sized nozzles.
Reduce plugging problems
Major plugging problems are found in all orchards that did not have good filtration, even those that get district water. There were also some “within-system” causes of plugging. Almost all plugging is from simple dirt or rust, as opposed to bacteria or algae. Recommendations are as follows:
- Always have a filter at the head of the system. The required mesh size depends on the microsprinkler flow rate, but 120 mesh is a starting point.
- Remove hose screen washers that are found at the head of hoses, and replace them with regular washers (after installing a filter at the head of the system). The hose screen washers often plug up and cause the hoses to have unequal inlet pressures.
- Be sure to thoroughly flush hoses after any hose breaks.
- Double check the type of fertilizer that is being injected, especially any “organic fertilizers”. Some of these can plug emitters. In any case, inject the fertilizers upstream of the filters. If the filter plugs up, it is better to have discovered the problem early.
- Clean the filters frequently. Install pressure gauges upstream and downstream. When the pressure differential (as compared to a clean screen) increases by 3-5 psi, it's time to clean the screen.
In some orchards, there is a big plugging problem caused by insects crawling into emitters after the water is shut off. Many of the new microsprinkler designs utilize a self-closing mechanism to prevent insects from coming into the nozzle.
We have gotten a reprieve with the rains and refilled reservoirs, but it is ever more important to make sure our irrigation systems are doing what they are supposed to be doing. Call your local Resource Conservation District and get information about a system evaluation. Contact numbers can be found at: http://www.carcd.org/rcd_directory0.aspx
- Author: Franz J.A. Niederholzer
Prunes are just past petal fall, but I've been shown several examples of bacterial canker and/or blast in the last week (see pictures below).
These symptoms are caused by infections of Pseudomonas syringae pv. syringe, a common bacteria found throughout orchards. Particularly in years with cold wet winters/springs, the bacteria enter prune trees -- and other stone fruit and almonds -- damaging scaffolds (bacterial canker) and buds (bacterial blast). The bacterial blast generally is not a major economic issue for growers. However, canker can spread rapidly and kill trees from the top down. Strangely enough, the rootstock is not affected by the bacteria, and damaged trees often show vigorous sucker growth.
Once the symptoms appear, there is little to be done. Don't remove damaged trees when symptoms first show. The trees may recover.
Bacterial canker is associated with:
- Soil related stress such as acid soils, shallow soils over hardpan, or coarse texture (sandy) soils with nematode (specifically ring nematode) pressure. The bacteria don’t enter the plant through the soil – at least they don’t harm the rootstock, but soil stresses predispose the tree to above ground infection.
- Low soil nitrogen availability
- Pruning during wet, cool weather.
- Rootstock selection. M2624 is most sensitive to bacterial canker, although M29C is susceptible as well. M40 is reported to be the rootstock that is least susceptible to bacterial canker.
What can be done to avoid bacterial canker?
- Avoid soil stresses listed above, wherever possible.
- Spot fumigate to control nematodes in coarse texture (sandy) soils
- Maintain adequate soil nitrogen levels
- Prune in late summer/early fall or late spring
- Plant on M40 rootstock
Also, Jack Dibble, retired UCCE entomology specialist, told me that, in his experience, trees with bacterial canker pressure showed dormant oil burn more readily than trees not under pressure from bacterial canker. If your block is at risk from bacterial canker (sandy soils, low N levels, nematodes, etc.), consider avoiding high dormant oil rates.
Prune trees showing bacterial canker symptoms (scaffold collapse, gumming). Sutter Co. April 8, 2011
Prune buds damaged by bacterial blast.