How to recognize Phytophthora and Pythium root rot in blueberries

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Root rot tends to be rare in Michigan blueberry plantings but where it occurs, it may be severe and responsible for visible plant decline. It is usually considered to be caused by the oomycete pathogen Phytophthora cinnamomi. However, we have recently discovered Pythium sterilum causing root rot in a Michigan blueberry field thought to suffer from Phytophthora root rot. Pythium has also been reported as a cause of blueberry root rot in Georgia, Florida, Oregon and British Columbia. Pythium spp. were actually much more common than Phytophthora spp. in declining blueberry fields. Oomycetes are fungal-like organisms which are more closely related to brown algae than to fungi. This probably explains why they like wet soils and are also called “water molds.” Root rot caused by oomycetes is not very common in Michigan but should be considered if blueberries are declining or dying in poorly drained soils or areas with standing water for several days. The use of woven weed cloth and black plastic mulch may also increase conditions for Pythium or Phytophthora root rot.

At this point, we believe Pythium and Phytophthora syptoms to be similar, i.e. yellowing or reddening of leaves and lack of new growth. Below-ground symptoms vary from slight necrosis of young rootlets to extensive necrosis with (partial) reddish-brown discoloration of crowns and main roots. Infected bushes are stunted and may die eventually. The pathogens live in the soil and produce swimming spores (zoospores) that infect the roots during conditions of high soil moisture. Soil temperatures between 68 and 90ºF (20 and 32ºC) promote disease development. Thick-walled chlamydospores or oospores are the primary overwintering structures and are released into the soil as the roots break down. These structures can survive for many years in soil. To diagnose Phytophthora or Pythium root rot, dig up a dying (but not dead) plant and send the entire plant or the crown and roots to MSU Diagnostic Services, 101 Center for Integrated Plant Systems, East Lansing, MI 48824 (tel. 517-355-4536).

If you receive a positive diagnosis for Phytophthora root rot, you can manage the disease by avoiding planting blueberries in poorly drained sites, particularly if the disease has occurred there previously. Also, improve drainage by tiling the field or growing plants on raised beds. Use effective fungicides, such as Ridomil Gold (drench) or phosphorous acids (e.g., ProPhyt or Phostrol) applied as a foliar spray or drench. Ridomil applications are usually made in the spring and fall when the pathogens are most active. ProPhyt or Phostrol can be applied on a monthly schedule through the season, starting in the spring. Advanced symptoms will not be cured; however, applications will halt the disease. In general, applying fungicides without correcting drainage problems is not very effective. Research in other crops, e.g., soybean, has shown that applications of calcium (particularly calcium formate) can reduce severity of Phytophthora root rot. This may be something to try although efficacy has not been confirmed in blueberries.

Dr. Schilder's work is funded in part by MSU's AgBioResearch.

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