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December Highlights

Things to watch for during December

From our experience of samples we have received during December in previous years in the OSU Plant Clinic, this page will highlight some problems that may occur this month. This month we include an extra category, parasitic plants, with some information on seasonal mistletoe. Also,find out about the many diseases of poinsettias in the PNW Plant Disease Management Handbook.

Fungal disorders

Two fungal diseases affecting lawn and turf grasses in fall and winter are pink snow mold (Microdochium nivale)

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pink snow mold

and red thread (Laetisaria fuciformis). Both diseases are favored by cool, wet, humid weather.

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red thread

Greenhouse crops can be susceptible to fungal diseases now that light levels are low, and cold outdoor weather conditions encourage condensation, high humidity, and reduced airflow inside. Gray mold on Poinsettia can affect all parts of the plants, including lesions on leaves, stems and bracts, and cankers on stems. Affected parts become covered in gray spores.

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Cool, wet conditions during spring can be conducive to the development and spread of blind seed, a fungal disease affecting grass seed, particularly of tall fescue and ryegrass (annual and perennial). Spores develop on the seed but infected seed is often difficult to distinguish from healthy seed, and the disease may only become apparent when subsequent germination is poor. The OSU Plant Clinic tests grass seed for the presence of this fungal pathogen.

Slime molds, which are fungal-like organisms that live in soil or thatch, appear during wet weather as a brown, gray or white mass of small round fruiting bodies on plants such as grass, making the leaves feel slimy. Despite the alarming appearance slime molds do not adversely affect plants and will decline after a couple of weeks.

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slim molds
slime molds

Bacterial disorders

Pruning and freezing temperatures in fall and winter, can cause damage to woody ornamentals, in the form of wounds, that predispose them to bacterial infections, including Pseudomonas syringae.

Management practices during the fall and winter can help mitigate problems later on. For example, several cultural control strategies are available for managing bacterial canker in cherry, including delaying dormant pruning, providing optimum nutrition and soil pH, and removing infected trees and branches.

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cherry bacterial canker

At this time of year when the nursery industry is increasing greenhouse stock, producers should be on the lookout for crown gall, caused by species of the bacterium Agrobacterium. We have had many samples lately in the OSU Plant Clinic, some with the obvious gall symptoms and some symptomless that still tested positive for this disease.

Coreopsis with Agrobacterium

Abiotic disorders

Many plants we receive throughout the year exhibit symptoms of disorders that are not due to pathogens. Unusual or abnormal environmental conditions that are significantly different from those required for normal growth cause stress and include:

  • unusually low or high temperatures
  • sudden changes of temperature
  • high temperatures with direct sunlight
  • too little or too much water
  • excessive or deficiencies of nutrients
  • too low or too high pH
  • chemical exposure 
  • mechanical wounding
  • poor glasshouse ventilation
  • ethylene toxicty in greenhouse crops

Parasitic plants

True mistletoe is a parasitic plant, or more correctly a hemi-parasite since it derives water, minerals and some nutrients from its host plant, but is able to photosynthesize on its own. In Oregon the hosts are mainly oak and juniper. Heavy infections can cause the host to weaken and grow more slowly.

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see also "The Plant Clinic Year"