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January Highlights

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

Watch for the new wilt disease of cucurbits was found by the Plant Clinic.  The soil-borne fungus Plectosphaerella was recovered from cucurbits which looked fine until nearing harvest, after which they rapidly wilted and died. The fungus does not cause much discoloration of the vascular system, unlike Fusarium or Verticillium. This fungus is known to cause disease in cucurbits such as squash, watermelon, pumpkins, cucumbers and muskmelons. Affected roots may be corky in appearance, and lack many root hairs or small roots. The fungus can persist on crop residue and in soil for several years, and rotation out of cucurbits of any kind for several years is recommended.

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cucumber, Plectosphaerella

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.

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phlox downy mildew

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.

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sedum powdery mildew

Red thread is noticeable in lawns now, especially those of poor vigor.  This is caused by a fungus and appears as yellow/brown circular patches in which pinkish threads and fruiting bodies of the fungus are interwoven.

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turf, red thread

Watch for Septoria leaf blotch in Willamette Valley wheat seedlings. This is disease can appear somewhat earlier than we would expect.

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wheat, septoria leaf blotch

Greenhouse crops are susceptible to black root rot, caused by a fungus that is found in field soils and commercial peat moss. Flat black root lesions are characteristic, although above ground symptoms are not unique and include yellowing leaves and stunting.

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lupin, black root rot

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.

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wall flower xanthomonas
Wall flower

stock xanthomonas

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.

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ajuga cucumber mosaic virus


Not too surprisingly, as plants go dormant and temperatures drop, we get fewer insect submissions at the Plant Clinic.


see also "The OSU Plant Clinic Year"