Leaf Spots - Cercosporoids, Guignardia & Phyllosticta & Septoria
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Infection shows first as a yellow spot on the underside of the leaf. Soon after infection occurs, the yellow-green area may be noted on the top surface of the leaf. As the spots enlarge in irregular patterns, they become slightly sunken and necrotic and turn purplish brown to purplish black. The spots continue to enlarge in a circular or irregular pattern and may eventually cover the entire leaf. The advancing margin remains yellow. Heavily infected leaves usually fall from the plant prematurely, especially if the infection started near the base of the leaf.
The Cercosporoid fungi primarily cause leaf spots and irregular blemishes. Depending on the Pseudocercospora species and orchid cultivar, leaf spots can be circular to nearly circular, reflecting the growth pattern of the fungal colony. Sometimes the circular blemishes are purple to black, with greater amounts of brown to black flecks forming as the spots enlarge. Other species of Pseudocercospora cause smaller, irregular blemishes generally in large numbers. A general mosaic pattern occurs on the upper leaf surface when large sections of the leaf are diseased.
The leaf undersides can be covered with dots, the sporing bodies, that often match the patterns on the upper leaf surface.
The first signs of Guignardia infection are tiny, dark purple, elongated lesions on either leaf surface. These lesions run parallel to the veins and elongate into purple streaks or diamond-shaped areas. Spots often merge to form large irregular lesions that may affect a large part of the leaf. With age, the center of the lesion turns tan. Raised, black sporing bodies develop in the affected area feeling like sandpaper. Affects mostly ascocentrums and vandas and their hybrids. This blight is also known as Phyllosticta; the names apply to two different sexual stages of the same fungus.
Spotting from Phyllosticta may start anywhere on the leaf or pseudobulb. The lesions are tiny, yellow and slightly sunken. As they enlarge, they become round to oval and more sunken, especially if the infection is on the leaves. With age, they turn tan to dark brown and develop a slightly raised, red to purple-black margin. Eventually, tiny black, raised spore structures develop in the center of the spots. Individual spots are about ¼ in across. Severely infected leaves may drop prematurely. This blight is also known as Guignardia; the names apply to two different sexual stages of the same fungus.
The tiny spots may start on either leaf surface as sunken, yellow lesions. They continue to enlarge, becoming dark brown to black, circular or irregular lesions. Spots may merge to form large, irregular patches on the leaf. Heavily infected leaves fall prematurely.
Spray with a suitable fungicide containing copper or quaternary ammonium compounds, Daconil or Cleary's 3336, or the some of the fancier fungicides like Heritage or Pagaent, following label instructions.
Good sanitation with good air movement. Reduce leaf wetness, water on the leaves may lead to infection. If the fungus is a continuing problem, monthly fungicide sprays may offer effective prevention.
Read More: The Dreaded Thai Disease - Phyllosticta Guignardia (Dr. Martin Motes),
Leaf Spotting Fungi on Cattleyas - Part 2, Cercosporoid Fungi (Sue Bottom)