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Lavender - Phytophthora root rot

Lavender - Phytophthora root rot
Lavender - Phytophthora root rot

Signs of Phytophthora crown rot and stem canker are wilting, decline and plant death. Photo of unhealthy rosemary roots infected with Phytophthora. Phytophthora, like Pythium, is 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 are sitting in puddles of water. Disease spores are dispersed via overhead irrigation or when water moves from infected plants to ones 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 disease prevention. No chemicals can cure infected plants and no biological control agents provide adequate control, only suppression.

  • 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.
  • 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