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Rosemary - Phytophthora root rot

Rosemary - Phytophthora root rot
Rosemary - Phytophthora root rot

Signs of Phytophthora root rot are wilting, decline and plant death. Note the dark colored, unhealthy roots on this rosemary plant. Phytophthora like Pythium is a a lower fungus favored by excess moisture and excess nitrogen fertility. Unlike Pythium, species of Phytophthora are more aggressive, more likely to be host specific, and less frequently found in greenhouses. The most likely source of origin is infected plants or in plant debris. This fungus- like organism can form swimming spores (zoospore) that move in water, so infections can be severe with prolonged irrigation or if plants re sitting in puddles of water. Disease spores are dispersed via overhead irrigation or when wate moves from infected plants to one nearby.

Phytophthora is very difficult to control because it produces several different types of spores that can cause disease. This walled oospores can survive between crops on plant containers, benches, floors and in potting media.

For management, focus on exclusion of this pathogen:

  • Purchase disease-free plants. Inspect incoming plants for signs of disease. 
  • Testing kits are available from agdia.
  • Only take cuttings from healthy plants. If cuttings are infected and under mist, whitish bloom of spores on the base of the stem will develop that can be spread by overhead watering.
  • Use a well drained potting mix and avoid overwatering.
  • Control fungus gnats to prevent spreading this disease.
  • Use clean containers.

There are very few fungicides labeled for herbs, therefore it is very important to prevent disease. Start with soilless growing media and avoid contaminating media with soiled hands, tools, or containers. Promptly remove diseased plants, avoid splashing water when irrigating and keep hose ends off the floor.

Suspicious plants can be diagnosed through your University diagnostic lab.

Fact Sheets:
Herb Bedding Plants Pest Management (with fungicide table)
Root Diseases of Greenhouse Crops
Disinfecting the Greenhouse