Powdery Mildew on Tomato

Published: April 1, 2015

While by no means a new disease, powdery mildew on tomatoes has been somewhat rare in Missouri until recently. The increase in the popularity of heated greenhouse and high tunnel tomato production has led to the creation of more ideal conditions for the fungi causing the disease to become virulent. The result has been several reports of this troublesome fungal disease in our state this year.

Powdery mildew is a disease of tomato leaf tissue that seldom kills the plant, but certainly has the prospect of drastically reducing yields by: a) invading the green leaf areas of tomatoes which otherwise will enable the plant absorb more solar radiation and produce more food through photosynthesis, and b) exposing tomato fruits to the sun (sun scald damage) after heavily infected leaves shrink in size. Pathogens that cause powdery mildew have a fairly narrow host range. This simply means the fungus that causes powdery mildew on tomatoes is not the same as the one responsible for powdery mildew on the likes of squash or pumpkin.

Traditionally, most cases of powdery mildew on tomato involved the fungus Leveillula taurica. It was first reported from California’s Imperial Valley in 1978 but subsequently been found throughout North America. Initial symptoms appear as irregularly shaped bright spots or “blotches” up to one-half inch in diameter on the upper surface of leaves.

As the spots enlarge, they eventually turn brown. Powdery, white colonies of mycelium (vegetative part of the fungus) later appears on the lower surface of the leaves as the disease progresses. Symptoms usually are not apparent on stems or fruits.

Recent, outbreaks of tomato powdery mildew, like this year in Missouri, have been traced to species of fungi in the genus Oidium. This disease was first reported in Canada in 1994 and then in the United States in 1996. Symptoms caused by Oidium appear as powdery, white colonies of mycelium on the upper surface of leaves. Yellowing, necrosis and defoliation can occur as the disease progresses.

Temperature and relative humidity are important environmental factors that affect powdery mildew severity. Unlike some other fungal pathogens, powdery mildew does not require standing water on leaves to be infective. High relative humidity along with moderate temperatures (60 to 77 degrees F.) favors disease development. Germination of conidia (seed like spores that land on a leaf surface and initiate new infection) and secondary infection readily occur under high relative humidity, and condia are readily windborne to spread the disease to the next leaf or plant.

The management of powdery mildew disease on tomatoes should follow integrated pest management (IPM) tactics. Start with clean, certified seeds or healthy, disease-free transplants. If the latter are purchased, inspect them thoroughly for early signs/symptoms of the disease. Producers who grow their own transplants should be especially vigilant for the disease in the transplant-rearing greenhouse.

The inoculum for the disease cannot overwinter outdoors under Missouri conditions. Therefore, tomato growers utilizing heated greenhouses or high tunnels can start with a “clean slate” each year if plant debris from the previous crop is eliminated. Soil preparation via deep plowing can help rid the production area of remaining inoculum on plant debris that might have been missed. Since moderate temperatures along with high relative humidity favors disease outbreak, the combination of high temperatures and low humidity can limit powdery mildew severity. Unfortunately, maintaining the latter combinations of environmental conditions in the early greenhouse or high tunnels can be very challenging. Therefore, chemical application might be required.

Unfortunately, the spray regimen recommended for the control of powdery mildew caused by L. taurica (please see Midwest Vegetable Production Guide) is not very effective for controlling O. neolycopersici and related species. A recent study reported sulfur applied as a wettable powder and potassium silicate (K2SiO3) to be the most effective chemicals to control powdery mildew of the Genus Oidium in growth chamber conditions. Under field conditions, hydrogen peroxide (H2O2) also was effective.

The authors of this article are skeptical concerning the use of sulfur in a greenhouse tomato operation because of its acrid, objectionable smell. The latter holds the potential of tainting at least the odor of tomato fruits, if not their taste. Additionally, there are questions concerning the validity of warranties on polyethylene plastic films used to cover greenhouses if sulfur is used inside.

This leaves potassium silicate as the chemical control of choice. The exact mode-of-action of this compound on powdery mildew has yet to be determined. However, recent research pointed to the fact that silicon acts to prevent fungal penetration through the formation of a “physical barrier” of some type.

t must be emphasized that potassium silicate is preventative in action and not curative. Therefore, regular application when environmental conditions are conducive to powdery mildew infection is needed if the disease is to be controlled.

In closing, powdery mildew represents another challenge to successful tomato production. However, through the use of IPM strategies reduction of yield from the disease can be kept to a minimum in most cases.