Powdery Mildew vs. Gray Mold on Tomato

Published: September 4, 2020

The incidence of fungal molds on greenhouse and high tunnel tomatoes has increased in recent years. While crop rotation would help to mitigate the problem, the high dollar value of tomato makes this an unattractive option to most growers. Therefore, disease management practices are extremely important for a successful crop. Accurate pest identification is the foundation on which a good disease management program is built. If a disease is not properly identified, the chances of selecting the correct management strategies are greatly reduced. Two troublesome tomato leaf disease that lately have been mistaken for each other are powdery mildew and gray mold.

Gray mold on tomato is caused by the fungus Botrytis cinerea which is a common pathogen of many plant species. Symptoms include light tan or brownish v-shaped spots on the leaves, beginning at their margins. Later, fluffy gray spores cover the surface of the spots and, ultimately, the leaf collapses and dies. The fungus also can cause cankers on stems and kill flowers and fruit. Gray mold is most virulent when the environment is cool (60-70° F) and the relative humidity is high (≥ 80%). Therefore, greenhouse and high tunnel tomatoes are especially at risk, especially early in the season before temperatures warm.

The epidemiology of powdery mildew is a bit more complex. There are several species of fungi that can cause tomato powdery mildew; symptoms vary with causal organism. The relatively recent outbreak of tomato powdery mildew in Missouri has been attributed to the fungus Oidium lycopersicum. Disease symptoms appear as powdery, white colonies of mycelium on the upper surface of leaves. Yellowing, necrosis and defoliation can result as the disease progresses.

The powdery mildew fungus produces airborne spores which land on leaves, germinate and infect the plant when favorable environmental conditions exist. The latter includes moderately temperatures (between 50 to 95° F) and high relative humidity. The increase in greenhouse and high tunnel tomato production in Missouri has led to the creation of more tomatoes being produced under ideal conditions for the disease to become problematic.

Cultural control of both gray mold and powdery mildew begins with keeping relative humidity within the greenhouse or high tunnel as low as possible. This can be a problem early in the season when temperatures are cool and fans are off, or sides dropped in the case of high tunnels. Later, when plants become large, lack of adequate air circulation within the leaf canopy aids to high humidity problems.

Encouraging good air movement by adequate plant spacing and leaf pruning helps to lower the humidity around the leaf surface. Additionally, good sanitation practices including the removal of all plant debris between crops helps to reduce inoculum of the diseases but will not prevent them entirely.

Additionally, strict sanitation is very important for the control of both diseases, since infected leaves carry inoculum that than be transferred to succeeding plantings. Therefore, tomato growers utilizing greenhouses or high tunnels should develop a "start clean, stay clean" attitude. Make sure that all plant debris from the previous crop is eliminated between crops. Soil preparation via deep plowing can help rid the production area of remaining inoculum on plant debris that might have been missed.

A number of fungicides have been recommended for tomato powdery mildew by the Midwest Vegetable Production Guide (https://mwveguide.org/). These include:

Correspondingly, the following fungicides are labelled for the control of gray mold on tomato:

Please note that Cabrio, Luna Sensation, Luna Tranquility and Switch are the only fungicides labeled for both powdery mildew and gray mold control.

Finally, genetic resistance is the easiest and least expensive way to control any disease. There are a few new tomato varieties that are advertised to have "intermediate resistance" to powdery mildew Examples include 'Climstar', 'Ducovery', 'Federik', 'Foronti', 'Geronimo', 'Granadero', 'Rebelski' and 'Touché'.

Unfortunately, there are no tomato varies that carry genetic resistance to gray mold.