Early and Late Blight Diseases of Tomatoes

Joe Nunez, Farm Advisor, Kern County

Early blight and late blight are two serious disease problems of tomatoes. Although early blight is common on potatoes in Kern County, it appears to be a rare problem on tomatoes in California. Late blight has not been a major problem for the last few years because California has been experiencing dry, warm springs. But under the right conditions these two leaf blights can quickly become serious problems. Being aware of them may help prevent them from causing extreme economic damage.

Both diseases are leaf blights that reduce tomato yields by killing the leaf and stem tissue and in severe cases infecting the fruit. Under ideal conditions these leaf blights can cause complete crop loss. Contrary to their names, early blight typically appears late in the growing season and late blight is usually found early in the season in the San Joaquin Valley. In other parts of the world their names are more fitting. To add to the confusion, these two diseases also look similar even though they are opposite from each other in many ways. Therefore, it is essential to understand the similarities and differences of these two problems to effectively manage them.

Late Blight: Late blight is the more devastating of the two blights. It can spread across a field in a matter of days and cause substantial loss of fruit and vines when conditions are right. Mild temperatures, moist or humid conditions, and lush, dense canopies favor late blight development. These are conditions that are more common during the early to mid part of the crop cycle.

A water mold fungus, Phytophthora infestans causes late blight. Phytophthora infestans is typically favored by cool, moist conditions. However, in the last decade new strains of P. infestans have developed naturally, and they are active at higher temperatures and drier conditions than the old strains. These new strains have virtually replaced the old strains in most parts of the country, including the San Joaquin Valley. Currently strain US-11 is the most common on tomatoes in California. US-1 used to be the common strain, but now we no longer see it. Unfortunately, these new strains of P. infestans are resistant to menfenoxan/metalaxyl (Ridomil), which had been very effective in the past for late blight control.

Late blight symptoms vary depending on environmental conditions. The typical conditions for late blight development are mild temperatures (65-75^oF) and 100% RH. But the new strains of late blight are active on potatoes and tomatoes in mid-summer, even under rather dry conditions. Typically, the lesions begin inside the plant canopy as small irregular shaped dark spots on the leaves. These spots will expand rapidly if high humidity or moisture is present. The lesions become black as they enlarge and a grayish mass of spores will develop on the underside of the leaves. The new strains also seem to infect the stems more than the older strains. The symptoms are black irregular lesions on the stem that will expand and often kill the terminal end of the stem or branch. Fruit can also be affected by late blight. The fruit will develop a brown discoloration but stay rather firm.

The late blight pathogen must survive between crops on living host tissue. In California this may be on volunteer tomato and potato plants, unharvested potato tubers, or even susceptible weed hosts such as hairy nightshade. It may also be brought into a field on infected transplants. Infections begin when spores from transplants or blown into a field germinate and start the cycle of plant infection. If conditions are ideal, late blight can spread rapidly across a field. Under dry, warm conditions late blight may be limited to localized lesions. The latter was observed in several tomato fields in the summer of 2001.

Control of late blight includes removing any nearby volunteer tomato and potato plants and nightshade. If using transplants, check that they are free of late blight before planting. Excessive nitrogen and water can produce lush, dense, and humid canopies that are ideal for late blight development. Fungicides can be used if there is a concern, but only as a preventive treatment and not as a curative. If conditions are conducive for late blight and/or late blight has been reported in the area, the interval between applications should be shortened. Scouting fields on a regular basis is important for late blight management because of how quickly it can start and spread.

Early Blight: Early blight begins as small black spots on the older, lower leaves. As the lesions expand, they produce a visible series of concentric rings that give the lesion the appearance of a "bull's eye" target. These lesions can also occur on the stems and fruit.

Early blight, caused by the fungus Alternaria solani, favors plants that are stressed, mature, or otherwise weakened. These conditions are more common towards the end of the crop season.

Early blight is able to survive on living hosts, on plant debris, or in the soil as spores. Again, volunteer tomato and potato plants should be destroyed. Proper crop rotation is important to insure that infected plant debris decomposes. Maintain good fertilization and irrigation practices to sustain the vigor of the plant until senescence. Fungicides are rarely needed on tomatoes for early blight control.

Consult the UC IPM tomato manual and pest management guidelines for more descriptions, photographs, and fungicide recommendations for either of these blight diseases.

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