Another Look at Tomato Powdery Mildew

Published: July 2, 2018

Powdery mildew continues to be a troublesome disease on greenhouse tomatoes here in Missouri. Unfortunately, more greenhouse and high tunnel tomato growers find themselves trying to manage a disease that ten years ago was nearly unheard of in our state. By means of this article, I would like to review the nature of powdery mildew and offer approaches to it management.

Powdery mildew is a tomato leaf disease that seldom kills the plant, but has the potential of drastically reducing yields. In Missouri, it appears to be exclusive on tomatoes grown "under cover" (i.e. greenhouses and high tunnels). Three different species of fungi can cause tomato powdery mildew: Leveillula taurica, Oidium neolycopersicum and Oidium lycopersicum. All produce airborne spores which land on leaves, germinate and infect the plant, given favorable environmental conditions exist.

Most cases of powdery mildew on tomatoes in the sub-tropical areas with arid to semi-arid conditions involve the fungus Leveillula taurica. Initial symptoms appear as 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 later appear on the lower surface of the leaves as the disease progresses.

Most outbreaks of tomato powdery mildew in Missouri have been traced 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 fungus responsible for powdery mildew infection produces airborne spores which land on leaves, germinate and infect the plant, given favorable environmental conditions exist. Moderately warm temperatures (between 50 to 95F) and high relative humidity are important environmental factors that promote powdery mildew severity. 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.

Tomato powdery mildew management should follow the principles of integrated pest management (IPM). Start with 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 powdery mildew cannot overwinter outdoors under Missouri conditions. 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.

Since moderate temperatures along with high relative humidity favors disease outbreak, the combination of very high temperatures and low humidity can limit powdery mildew severity. Unfortunately, maintaining the latter combination of conditions in greenhouses or high tunnels can be very challenging. Therefore, chemical application might be required.

A study conducted several years ago reported sulfur (WP) and potassium silicate to be the most effective chemicals for control of Oidium in growth chamber conditions.

Sulfur has been used for many years to control powdery mildew on a number of species, including tomato. Sulfur can easily cause phytotoxicity, especially under hot and humid conditions typical of a greenhouse. Therefore, care must be taken when it is used. Like most fungicides, it is a preventative and must be applied before the disease appears.

Several growers have reported good powdery mildew suppression by vaporizing elemental sulfur in commercially-available "sulfur evaporators". These devices heat the sulfur to a controlled temperature causing it to change into an isotope of elemental sulfur which is distributed via air currents throughout the greenhouse. In the process, the leaves of tomato plants are coated with elemental sulfur which acts as a deterrent to powdery mildew spore germination. Evaporators are rated in size according to the square footage of greenhouse space they can service. Good horizontal air movement is needed for them to be effective. Carefully follow the instructions that accompany sulfur evaporators to avoid plant damage.

Potassium silicate (available commercially as Sil-MATRIX®) is labeled as a powdery mildew control agent. The exact mode-of-action of this compound 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. Like sulfur, potassium silicate is preventative in action and not curative.

Regalia® is a biofungicide with a novel mode-of-action. An extract of the giant knotweed, Regalia's active ingredient is reported to enhance a plant's ability to protect itself against disease attack. It is labeled for a number of tomato foliar diseases including powdery mildew. Taegro® is another biofungicide labelled for powdery mildew control on tomato.

Paraffinic oil (e.g. Ultra-Fine® Oil) has been shown to eradicate mild infestations of powdery mildew, even though its primary use is as an insecticide. As with sulfur, care must be taken when applying oils because of their phytotoxic tendencies.

A number of fungicides have been recommended for tomato powdery mildew by the Midwest Vegetable Production Guide (https://ag.purdue.edu/btny/midwest-vegetable-guide/Pages/default.aspx). These include:

• (a) Aprovia Top®; 0-day PHI;
• (b) Cabrio®; 0-day PHI
• (c) Inspire Super®; 0-day PHI
• (d) Priaxor®; 0-day PHI
• (e) Quadris 2.08EC®; 0-day PHI
• (f) Quadris Opti® at 1.6 pts. per acre. 0-day PHI
• (g) Quadris Top®; 0-day PHI
• (h) Quintec®; 3-day PHI
• (i) Rally 40WSP®; 0-day PHI
• (j) Switch®; 0-day PHI
• (k) Vivando®; 0-day PHI

Unfortunately, many of the fungicides recommended for powdery mildew control caused by Leveillula taurica have not proven themselves to be equally effective against Oidium lycopersicum.

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 by the companies that market them. Examples of varieties include 'Climstar', 'Ducovery', 'Federik', 'Foronti', 'Geronimo', 'Granadero', 'Rebelski' and 'Touché'.