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Things to watch for during January From our experience of samples we have received during January in previous years in the OSU Plant Clinic, this page will highlight some problems that may occur this month. Fungal disorders Downy and powdery mildews are are caused by different fungal organisms and cause different symptoms. Downy mildew in greenhouse plants is a problem in cool, wet conditions and high relative humidity, where leaves remain wet for long periods. Many different plants can be affected including snapdragon and phlox. click on image for more information
The fungi causing powdery mildew in greenhouse plants have a wide host range, and may be present on different plants in the same greenhouse. Spores produced are easily moved by air movement and water splash. The pathogen survives in the greenhouse in weed hosts or on crops. Powdery mildews are favored by high relative humidity, moderate temperatures, and low light intensities. Epidemics that seem to develop overnight are often the result of undetected low level infections that have spread spores throughout the greenhouse. click on image for more information
Bacterial disorders Herbaceous ornamentals such as wall flowers and stocks are susceptible to a bacterial blight that persists in seed and affects the water transport system. It is caused by the bacterium Xanthomonas campestris and plants may show no symptoms until small seedlings suddenly wilt, or there may be necrosis or water soaked areas on stems and lower leaves. The bacteria are rapidly spread by water splash. click on images for more information
Abiotic disorders Prolonged and very low temperatures cause winter injury especially to non-native plants that are marginally hardy in a particular geographic area. click on images for more information
Winter sunscald is a common winter injury that affects woody ornamentals in very cold or extended periods of very cold weather. Sun can cause the temperature in plant tissue to rise, falling again rapidly at sunset, sometimes with the formation of ice. Plants cannot acclimatize to such rapid temperature change and tissue may be damaged or killed. Leaves may die, and bark may discolor, crack open, or separate from the tree. click on image for more information
Viral disorders Cucumber mosiac virus, first found in cucumber, is commonly found in many different greenhouse plants. Transmitted by aphids, this virus produces a variety of symptoms including, leaf mottling, yellowing, mosaic, stunting and distortion. click on image for more information
see also "The Plant Clinic Year"
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May 14, 2009
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Oregon State University Plant Clinic 
