Several years ago in this publication, we brought to light the fact that powdery mildew was becoming a troublesome disease on greenhouse tomatoes here in Missouri. Unfortunately, management of the disease continues to be a challenge for tomato growers this year thanks in part to the warm weather that started unusually early and the limited effectiveness of control measures. This has resulted in a negative impact on Missouri’s greenhouse tomato industry. This article reviews the nature of powdery mildew and several new approaches to its management.
Powdery mildew is a tomato leaf disease that seldom kills the plant, but has the prospect of drastically reducing yields. 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 of the recent 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.
Both of the above fungi produce airborne spores which land on leaves, germinate and infect the plant, given favorable environmental conditions exist. Moderately warm temperature (conidia of both fungi can germinate between 50° to 95°F) and high relative humidity are important environmental factors that affect powdery mildew severity. The increase in greenhouse and high tunnel tomato production in Missouri has led to the creation of more ideal conditions for the disease to become virulent.
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 disease cannot overwinter outdoors under Missouri conditions. Therefore, tomato growers utilizing greenhouses or high tunnels should start with a "clean slate" each year making sure that all 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 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.
Most of the fungicides recommended for the control of powdery mildew caused by Leveillula taurica have not proven themselves to be equally effective for Oidium lycopersicum and related species control. 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. Under field conditions, a mixture of hydrogen peroxide and crop oil was also found to be effective.
Sulfur has been used for many years to control powdery mildew on a number of species. Like most fungicides, it is a preventative and must be applied before the disease appears. Sulfur products used for the control of powdery mildew on vegetables usually contain a surfactant to make them more effective. Sulfur can easily cause irritation and also phytotoxicity, especially under hot and humid conditions. Therefore, care must be taken when it is used.
Potassium silicate (available commercially as Sil-MATRIX®) is relatively new 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 new biofungicide with a novel mode-of-action. An extract of the giant knotweed, Regalia’s active ingredient is reportedly 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, gray mold (Botrytis), bacterial speck and spot, early blight and late blight. Preliminary studies at Lincoln University from a weekly spray of Sil-Matrix and Regalia on cucumber powdery mildew indicated slowing down of the disease when compared with the non-treated control, but not sufficient suppression of the disease.
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 new compound not yet available in the United States holds hope for tomato producers who are battling powdery mildew. A recent research article from Spain reported that liquid bioassimilable sulfur (NATURDAI S-SYSTEM) reportedly was effective in powdery mildew control. It was applied both as a foliar spray and as a soil drench. The authors suggested bioassimilable sulfur acts as a preventative by enhancing tomato’s resistance to the mildew pathogen. Additionally, applied as a foliar spray, the compound demonstrated curative properties.
A number of fungicides have been recommended for tomato powdery mildew in the Midwest Vegetable Production Guide. Authors of this article would welcome feedback on the performance of these chemistries under Missouri conditions. The fungicides include:
Finally, genetic resistance is the easiest and least expensive way to control any disease. There is a limited number of new tomato varieties that exhibit intermediate resistance to powdery mildew. Examples of varieties with intermediate resistance include 'Geronimo', 'Foronti', 'Ducovery' and 'Touché'.
REVISED: February 21, 2017