Diseases of tomato (
Powdery mildew
Tomato powdery mildew is caused by two different fungi, Leveillula taurica (Oidiopsis sicula) and by Erysiphe (Oidium lycopersicum). The imperfect stages (Oidiopsis and Oidium) of both fungi have been observed in greenhouses in Arizona. The perfect or sexual stages of these fungi have not been observed.
Leaves infected with Oidiopsis sicula develop irregular, bright yellow blotches (photo 1), and severely affected leaves die but seldom drop. Spots of dead tissue, sometimes surrounded by a yellow halo, eventually appear in the blotches (photo 2). There are no lesions on stems or fruit. A light gray mycelium may develop on the lower leaf surface of infected older leaves. Infections usually are not severe on tomato in Arizona greenhouses, but can cause defoliation in greenhouse bell pepper. The fungus infects weeds and crops in the solanaceous family such as pepper as well as other hosts. Spores are carried by wind, air currents or workers. High relative humidity favors disease development while free water from overhead irrigation may discourage disease. Mild temperatures favor infection while higher temperatures hasten the death of infected leaves.
Oidium lycopersicum infections are more similar to those caused by other powdery mildew fungi on other kinds of plants such as cucumber. White spots consisting of mycelium and spores develop on the top side of the leaves (photo 3 and 4) and stems (photo 5). They are usually conspicuous even when small. Yellow lesions do not develop with infections by this fungus, but large areas of the leaves may be affected.
Removing old leaves from the bottom of plants reduces disease from both fungi, often to the extent that no other control measure is needed. If symptoms develop early, multiple, early applications of wettable sulfur or dusting sulfur will prevent new infections. Sulfur should be used with caution on young foliage at temperatures above 90°F; read and follow label directions. Applications of other contacts such as potassium bicarbonate products and oil sprays will kill the existing mycelium and spores on the leaf surface and are most effective when disease is in early stages.
Links for more Information:
http://www.hort.uconn.edu/ipm/veg/htms/pmildtom.html
http://www.ipm.ucdavis.edu/PMG/r783100411.html
http://edis.ifas.ufl.edu/PP109
Images of Oidiopsis taurica:
http://www.ipm.ucdavis.edu/PMG/r783100411.html
Images of Oidium lycopersicum:
http://www.avrdc.org/LC/tomato/powderymildew01.html
http://vegetablemdonline.ppath.cornell.edu/Images/High_Tunnels/High_Fig11.jpg
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February 14, 2006