In the last couple of months I have received many calls asking, “Why did my tomato plants die this year?” Although, there are many reasons tomatoe plants die or fail to produce fruit, such as: weather, diseases, pest, improper watering, or poor soil conditions. There is a relatively new disease that maybe causing the die-off of local tomato plants.
Since the mid 90’s, tomato spotted wilt virus (TSWV) has become one of the most serious diseases of tomatoes in our area. Always deadly, there is no way to treat TSWV and infected plants should be pulled up and discarded.
Currently, the virus is particularly damaging to pepper and tomato crops and is rapidly and specifically spread by thrips. This article provides basic information about TSWV biology and detection, and an integrated pest management (IPM) approach for tomatoes.
Detection
Symptoms of TSWV infection in tomatoes are highly variable, and depend on the stage of growth when plants are infected, the cultivar, and environmental conditions.
TSWV symptoms can also be confused with those caused by other viruses (e.g., curly top virus and Alfalfa mosaic virus).
Thus, it is important to confirm suspected TSWV infections by using a diagnostic immunostrip test. Results of these tests are quick (5-10 min.), and kits are available from companies such as AgDia (www. agdia.com) and EnviroLogix (www.envirologix.com).
Symptoms and Signs
• Infection at seedling or transplant stage. Visually detecting the virus at this stage is the most challenging. Early infection of seedlings leads to extremely stunted growth. Infected plants may die. Leaves may curl, wilt, and develop yellowing/bronzing and necrotic spots or rings. TSWV infection of a transplant.
• Infection at early vegetative stages. Initial symptoms are leaf bronzing and wilting, followed by necrotic leaf spots and some degree of vein, stem, and petiole necrosis.
Often the entire plant is off-color and leaves may have a crumpled appearance. Green fruit may develop bumps and show diagnostic concentric rings. As the tomatoes ripen on plants infected during early stages of growth, fruit may be distorted and have necrotic rings or etchings.
• Infection at flowering through fruit set stage. Symptoms may develop in only one or two shoots. Initial symptoms include leaf curling, pale green to yellow discoloration, and purpling. Fruit may be bumpy, deformed, and develop ringspots, blotches, and necrosis. TSWV symptoms at all growth stages can be confused with curly top and alfalfa mosaic. However, if necrotic spots and rings develop on leaves, followed by necrosis and dieback of entire leaves or shoots, and especially if fruits have the symptoms described immediately above, TSWV is more likely the cause.
Comments on the Disease
Tomato spotted wilt virus is transmitted by various species of thrips, including the western flower thrips, Frankliniella occidentalis, the onion thrips, Thrips tabaci, and the chili thrips, Scirtothrips dorsalis. Tomato spotted wilt virus also infects the thrips vector. Nymphs that acquire the virus by feeding on infected plants will retain the ability to transmit it for the remainder of their lives. Tomato spotted wilt virus cannot be passed from infected females through the eggs.
The virus has an extremely wide host range, including many weeds and ornamentals as well as crop hosts. It is one of the few plant viruses with a host range that includes dicots and monocots (e.g., tomatoes and onions). Recent outbreaks have occurred in the San Joaquin Valley where they are believed to be associated with nearby infested crops or weeds.
Management
Management of tomato spotted wilt is generally managed through plant resistant varieties and control of western flower thrips and onion thrips when the virus is detected early.
Before the Growing Season
• Plant resistant tomato varieties with the Sw-5 gene. Resistant varieties generally do not require insecticide applications for thrips to control tomato spotted wilt.
• Use virus- and thrips-free transplants from greenhouses that manage thrips and inspect transplants.
• Manage thrips on transplants before planting.
During the Growing Season
• Monitor for thrips and tomato spotted wilt symptoms.
• If thrips are present and symptoms are observed, manage thrips to minimize the spread of the virus within the garden.
• Consider removing infected plants at the seedling stage.
• Control weeds in and around garden.
After the Growing Season
• Promptly remove and destroy old tomato plants and other host crops after harvest.
• Control weeds and volunteer plants near next year’s tomato planting area.
So, next year when selecting which variety of tomatoes to grow, it is important to understand disease resistant codes for tomatoes.
When a cultivar has been developed that is tested and confirmed to be resistant to a particular disease, it is given a designated letter (after its variety name) donating that disease.
Multiple letters after a tomato variety name indicate that that type of tomato is resistant to more disease (all those indicated by the letters listed.) So the disease resistance codes are a helpful tools in your hunt for disease resistant tomatoes for your garden.
An example
Tomato variety Abe Lincoln Improved is labeled with letters VFNASt. Following the tomato disease resistance code listed here, those letters indicate that it is been bred to be resistant to verticillium wilt, fursarium wilt, nematodes, Alternaria, and Stemphylium (gray leaf spot).
Tomato disease
resistance codes:
V Verticillium Wilt
F Fusarium Wilt
FF Fusarium, races 1 and 2
FFF Fusarium, races 1, 2, and 3
N Nematodes
A Alternaria
T Tobacco Mosaic Virus
St Stemphylium (Gray Leaf Spot)
TSWV Tomato Spotted Wilt Virus
Keep in mind that a tomato’s resistance label is simply an indicator. How the variety performs will depend on several different factors, including its environment, rainfall, soil content, and care.
Sources:
How to Choose the Most Disease Resistant Tomatoes for Your Garden, http://www.tomatodirt.com/disease-resistant-tomatoes.html
Tomato Spotted Wilt Disease, Detection, Epidemiology, and Integrated Pest Management (IPM), Departments of Plant Pathology and Entomology, UC Davis and UC Cooperative Extension