THE COTTONBALL DISEASE OF CRANBERRY IN WISCONSIN: OCCURRENCE, SYMPTOMS, AND DISEASE PROGRESS

Monilinia oxycocci, the causal agent of cottonball, has been isolated from symptomatic shoots or fruit of the cultivars Ben Lear, Bain-McFarlin, Pilgrim, Searles, and Stevens and from symptomatic berries growing in the wild. Cottonball has been observed in all cranberry-growing regions of Wisconsin. Symptoms and disease progress were monitored in 1986 and 1987 in a commercial planting in central Wisconsin. In 1987, apothecia first were observed at the time of bud break (13 May). Tip blight symptoms first were observed on 10 June 1986 and 3 June 1987. Initial symptoms appeared as flagging of upright shoots followed by necrosis of petioles and midribs of leaves just behind the shoot tips. Necrosis later extended to the leaf laminae in a characteristic tan-colored, V-shaped pattern. White ectostromata, bearing conidia, formed on the crooks of infected shoots and peduncles of flowers and were observed one week following initial symptoms. Conidium production coincided with bloom. Approximately 4 wk after initial symptoms were observed, entire shoot tips had become necrotic and M. oxycocci could no longer be isolated; however, it could be isolated readily from infected, symptomless berries. By harvest, sclerotia had developed in up to 50% of infected berries. Further development of M. oxycocci was not observed in fruit in which sclerotia had not formed.