Primrose (Primula spp.)-Botrytis Blight

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Cause Botrytis cinerea, a fungus that colonizes dead, dying, and wounded plant parts. From these infections they can attack healthy tissues. A moist, humid environment is ideal for pathogen sporulation and spread. Conidia may come from many sources in and outside the greenhouse. It is found everywhere plants are grown and has a wide host range. Flower petals are most susceptible at any stage of development.

This fungus has been shown to be seedborne. Seed treatment did not prevent some plants from being colonized even when grown under sterile conditions. The fungus may grow systemically in the plant without producing symptoms until flowering and/or senescence.

Symptoms Leaves rot, especially those near the soil surface. Fading flowers and rotted leaves develop a gray fungal growth.

Cultural control

  • Remove all dead and dying plant parts (particularly blossoms) on and around plants and take them out of the greenhouse. Many growers use closed containers to minimize the amount of conidia.
  • Maintain a relatively dry greenhouse environment by keeping humidity below 90%, increasing spacing between plants for good air circulation, and taking care not to splash water on foliage during watering. Heating the greenhouse at night or venting around sunset may be necessary.
  • Spacing plants also allows better light penetration, reducing senescence of the lower leaves and thus helping reduce the amount of highly susceptible plant tissue.
  • Place fans in greenhouse for better air circulation.
  • Seed treatment has not been investigated and is not recommended but cleaning seed of all plant debris may be useful.

Chemical control Best when used with cultural controls. Tank-mix and/or alternate products with different modes of action to prevent the buildup of resistant fungi. Limit the use of any one group during crop production.

  • Astun at 10 to 17 fl oz/100 gal water. Group 7 fungicide. 12-hr reentry.
  • Chipco 26019 FLO at 1 to 2.5 quarts/100 gal water. Group 2 fungicide. 12-hr reentry.
  • Decree 50 WDG at 0.75 to 1.5 lb/100 gal water. Group 17 fungicide. 12-hr reentry.
  • Pageant at 8 oz/100 gal water. May see flower damage at 12 oz or higher. Do not use with organosilicone-based adjuvants. Group 7 + 11 fungicide. 12-hr reentry.
  • Phyton 27 at 1.3 to 2 oz/10 gal water. Group M1 fungicide. 48-hr reentry.
  • Terraguard SC at 4 to 8 fl oz/100 gal water. Group 3 fungicide. 12-hr reentry.

Biological control

  • Prestop (Gliocladium catenulatum strain J1446) at 0.33 oz/5 gal water. Do not use with other products in the tank. 4-hr reentry. O

Reference Barnes, S.E., and Shaw, M.W. 2003. Infection of commercial hybrid primula seed by Botrytis cinerea and latent disease spread through the plants. Phytopthology 93:573-578.