Weather conditions are ideal for Phomopsis diseases (2006)

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.  

A number of small fruit crops are affected by fungi belonging to the genus  Phomopsis. For instance, Phomopsis vaccinii causes twig blight and cane canker on blueberries as well as twig dieback and fruit rot on cranberries; Phomopsis viticola causes cane and leaf spot and rachis infections and fruit rot on grapes; Phomopsis obscurans causes leaf blight and fruit rot in strawberries. Yet other Phomopsis species cause tree fruit diseases, demonstrating that this fungal genus is versatile and well adapted to fruit crops. In particular, Phomopsisspecies like cool,  rainy weather, such as we have been experiencing for the past week. I therefore would like to pay special attention to Phomopsis diseases at this time,  espcieally since Phomopsis twig blight symptoms have already been reported in blueberriesin Ohio and may be appearing in Michigan soon. In grapes, small brown spots surrounded by yellow halos indicative of Phomopsis have been seen on young leaves

All Phomopsis species are “cousins” and tend to behave rather similarly. As a general rule, they:

  • Overwinter in infected perennial plant parts, particularly canes and twigs.  In herbaceous perennials, they overwinter in old leaves and fruit remnants. Old wood may be a source of spores for several years.
  • Are often introduced in a field on the planting material (dead twigs and lesions may be visible on young plants).
  • Produce pear-shaped fruiting bodies (visible as brown to black pimples in infected plant parts) with tiny spores that are exuded in gelatinous masses and dispersed by rain splash and irrigation water.
  • Don’t have a sexual stage or the sexual stage is rare, resulting in few or no airborne spores.
  • Spore dispersal is local (usually within 3 feet from the source, maybe a bit further in case of wind-driven rain)
  • Rain contributes significantly to spore production, dispersal, and infection. Longer wetness durations (24-72 hours) are ideal for infection. Frost injury and wounds may predispose tissues to infection.
  • Are active during most of the growing season. Phomopsis species on grape and blueberry are known to release most of their spores in spring and early summer, which is an important time for disease control.
  • Have one or more infection cycles per season. Phomopsis on grape does not generally produce spores on newly infected tissues so that overwintering inoculum is all that is available. However, the Phomopsis species on blueberry, cranberry, and strawberry do, leading to multiple infection cycles in these crops.
  • Prefer cool to moderate temperatures (59-75ºF).
  • Young tissues are most susceptible to infection.
  • Cold weather may slow down growth of young tissues and increase drying time if wet, therefore prolonging the susceptible period.
  • Infection may be latent for some time, particularly in fruit. Even on vegetative tissues, the infection may not become visible until 2-4 weeks after infection.

Control of Phomopsis diseases is best accomplished by a combination of sanitation (removal and destruction of diseased plant parts) and use of effective fungicides. Do not underestimate the importance of selective pruning in blueberries and grapes and removal of old leaves and fruit in strawberries and cranberries, even though these may be labor intensive. After a heavy outbreak of disease, it will be the main thing that you can do to regain control of the disease. As far as fungicides are concerned, availability varies by crop.  In general, good protectants are Dithane/Penncozeb (grapes and cranberries),  Bravo (blueberries and cranberries), Ziram (grapes, blueberries), and Captan (blueberries, strawberries, grapes [where not restricted]). Other fungicides that have performed very well against Phomopsis diseases have been Indar, Topsin M (+ Captan or Ziram), and Pristine in blueberries; Abound, Sovran, Pristine,  and ProPhyt in grapes; and Topsin M (+ Captan), Cabrio, Nova, and Pristine in strawberries. Dormant sprays of Lime sulfur, sulfur (e.g. Sulfur 6L), and copper (e.g., Cuprofix), are useful in reducing the amount of overwintering inoculum in grapes and blueberries and thereby disease pressure during the growing season.

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