Fruit Rot

Notes About Cranberry Fruit Rot:

photo of two Maine cranberries; one of them is green and healthy but the other one is orange and crumpled some from fruit rot infection
A healthy cranberry (left) beside one infected by cranberry fruit rot (as seen in late August in Maine).
  • Fruit rot is the most prevalent disease problem that cranberry growers face from season to season, and there are at least 13 species of pathogenic fungi that are associated with it, and several more species that are suspected to be involved with it in one way or another.
  • Some species of fruit rot fungi are believed to cause exclusively “field rot” (rot that occurs in the field before harvest), some are considered to cause exclusively “storage rot” (rot that occurs post-harvest), while still others appear to contribute to both field rot and storage rot, such as “Blotch Rot” which is pictured below.
  • The fungal spores land on the blossom (by wind and wind-driven rain) or small developing fruit and–if there is a suitable layer of moisture present for six to eight hours–the plant tissue may succumb to fungal infection.
  • If conditions are favorable and the cultivar is susceptible, berries may show rot symptoms one week after fungal infection (unfavorable conditions just means it takes longer). Interestingly, in the case of Blotch Rot pictured above, university of Wisconsin research has found that 40 to 60% of rotten cranberries from a given site will typically contain the blotch rot fungus.  However, this species of fungus has also been found in 10 to 30% of healthy cranberry fruit, indicating that its presence does not always lead to disease expression.
  • Excess nitrogen can increase fruit rot.
  • Failure to get good fruit rot control is usually not due to fungicide resistance! Poor fungicide timing is often the more likely culprit.
  • Holding a ‘Late-water flood’ significantly reduces the level of fruit rot inoculum on a bed and it will improve berry ‘keeping quality’ as well. Lower fungicide rates (as low as the minimum rates on product labels permit) can usually be used, as well as fewer fungicide applications, in the year the flood is used. In Massachusetts, researchers have found that the benefits continue somewhat into the following year as well such that fungicide applications and rates can still be reduced without a discernible loss in fruit quality.
  • One’s first fruit rot fungicide application should be applied no later than when half the cranberry flowers are open (i.e. 50% in bloom) and ideally before that point (as early as 10% of the flowers open).
  • Once berries start to ‘size up,’ fungicides aren’t much help (any infections have already occurred by that point).
  • One or two fruit rot sprays appears to be sufficient for most of Maine for controlling cranberry fruit rot; still seeing a need for (and recommending) three to four fungicide applications in Massachusetts, however, at 7 to 14 days apart.

Further/Recommended Reading:

Cranberry Fruit Rot (pdf) (University of Wisconsin-Extension)