Tomato Diseases Watch

Tomato Diseases to Watch for in 2012

TSWV causes ringspots on young fruits and chlorotic spotting on ripe fruit.


Tomato spotted wilt virus (TSWV) is a major pathogen for tomatoes, peppers, peanuts and tobacco in the U.S. It has over 2,000 known hosts, many of which are asymptomatic weeds. In Utah, the virus is transmitted by western flower thrips and onion thrips. Thrips have to acquire the virus when they are young (larvae) to be able to transmit the virus as adults. Therefore, adult thrips have to lay eggs on infected plants because thrips nymphs cannot fly.

Infection of plants occurs early in the season. Symptoms on tomato leaves are necrotic, brown lesions. Plants are often stunted. Necrotic, brown ringspots occur on immature fruit and chlorotic spots occur on ripe fruit. Similar symptoms can also be observed on peppers. TSWV is managed by using resistant tomato varieties when available. Names of resistant varieties can be found on the Internet. Reflective mulch (silver colored) can reduce the number of thrips flying to the plants. Thrips control with insecticides is the most common way to prevent infection. Insecticide applications can be challenging due to thrips’ ability to quickly develop resistance to some insecticides, as well as their ability to hide in buds, folded leaves, and other plant parts.

Late blight causes brown-black lesions on leaves and stems, and brown, firm lesions on fruit.


Late blight is caused by Phytophthora infestans. Infection occurs during wet or humid conditions at temperatures of 60-70°F. Necrotic lesions develop on leaves and can rapidly encompass the entire leaf. Infected leaves can fall off within a few days, leading to rapid defoliation of plants. Necrotic lesions can also develop on stems, and spread to fruit, forming firm brown spots.

Under humid conditions, mycelium develops that is visible with a hand lens. It is sometimes necessary to incubate symptomatic tissue for a few days with a moist paper towel in a Ziploc bag at room temperature to induce growth of mycelium.

Use of resistant cultivars such as ‘Mountain Magic’, ‘Plum Regal’, ‘Legend’, or ‘Red Pearl’ can significantly reduce disease incidence. The use of resistant varieties is especially important if potatoes, which are also susceptible, are grown nearby. Potato cull piles should be eliminated because the pathogen can overwinter on the culls.

To reduce the amount of inoculum present throughout the growing season, remove dead leaves, vines, and infected fruit and burn them or throw them in the garbage. Infected plant material should not be composted as temperatures in the compost pile often do not get hot enough to kill the pathogen. Avoid overhead irrigation to reduce leaf wetness and space plants widely to improve air flow. Preventive or protectant fungicides include chlorothalonil, azoxystrobin, or copper. Copper is acceptable for organic production.

Left: Early blight causes necrotic lesions on tomato leaves that often have concentric rings and a yellow halo.
Right: Infections on fruit occur on the calyx end.


Early blight is caused by the fungus Alternaria solani. This pathogen causes necrotic lesions on tomato leaves that often have concentric rings and a yellow halo. Lesions on stems may also have concentric rings.  Infections on fruit occur on the calyx end.

Other susceptible vegetables besides tomatoes include pepper, eggplant, potato, and weeds in the nightshade family. Infections occur during warm (75-84°F)and humid conditions. Under ideal conditions, Alternaria spores can germinate and infect plant tissue within one hour of landing on the host surface. Most infections occur on older leaves.

Alternaria can be seedborne and it is important to use clean seed. Growers who want to use seed from their own tomato plants should only collect seed from healthy plants. Removal of weeds in the nightshade family as well plant debris reduces the amount of inoculum and disease incidence. As with late blight, avoid overhead irrigation to reduce leaf wetness and space plants widely to reduce early blight incidence. When necessary, fungicides containing chlorothalonil, azoxystrobin, or boscalid can be used.

-Claudia Nischwitz, Plant Pathologist