• Print
• Share

Calyx End Rot: A Rare Disease

Occasionally, minor apple diseases can cause problems for some growers. For example, calyx end rot, a minor disease in apples, was observed in a few Ontario, New York and Michigan orchards last year. The disease rarely occurs but probably caused up to 15-20% fruit loss in the few infected orchards. The susceptibility of apple cultivars to calyx end rot is not known, however, this disease has been documented to caused losses in McIntosh, Cortland, Paulared, Rome Beauty, Delicious, Honeycrisp, Milton and Macoun.

Symptoms first appear as a small grayish brown lesion on the blossom end of developing fruit and are often not noticed until June or early July (Figure 1). Infection actually takes place about a month earlier, around petal fall. As the lesion expands around the calyx, the adjacent fruit tissue becomes soft and slightly sunken. Often a red ring develops around the lesion, which makes infected fruit standout from the green healthy non-infected fruit.

Figure 1. Light brown lesion on calyx end of infected immature McIntosh apples.

Based on the biology of the causal agent, there is a very narrow window for infection and disease development. Calyx end rot is caused by the fungus Sclerotinia sclerotiorum which, can infect and cause disease in over 360 species of plants including many common weeds. The pathogen over-winters as hard, black sclerotia just under the soil surface usually around the base of infected weeds. In the spring, when soil moisture remains near field capacity for 10 days to several weeks and temperatures remain cool (11- 20^OC), sclerotia germinate and produce tiny light brown mushrooms called apothecia. The apothecia have inverted cup-like caps that contain ascospores, which are forcibly ejected into the environment. These spores do not infect fruit directly but rather infect senescent, withered tissues such as flower petals during petal fall. The petals are colonized by the fungus, which gives the pathogen the energy to invade sepals and eventually the calyx end of the fruit. There is very little potential for further infection to occur after petal fall, since, the fungus does not produce secondary spores and therefore will not spread from infected apple to non-infected apple during the growing season. Once inside the calyx end, the fungus eventually colonizes the adjacent fruit tissue causing the rot symptoms observed. Infected fruit usually drop prematurely and the fungus may infect susceptible weeds it comes directly in contact with on the orchard floor. Eventually sclerotia are formed either on the colonized fruit or infected weed.

Last season, only the fruit produced by the secondary blooms were observed to be infected, whereas the fruit produced by the king and tertiary blooms in the same cluster were not infected, indicating the small window when environmental conditions were conducive for infection. A long wet period leading up to and during pedal fall of the secondary blooms last year was perfect for sclerotia germination, apothecium development, spore release and pedal infection.

The few orchards in Ontario observed to have calyx end rot also had an abundance of dandelions growing along the orchard floor. Dandelions are a very good host for the pathogen; in fact, S. sclerotiorum has been investigated as a potential biocontrol agent for dandelion management. The thick and dense canopy created by large populations of weeds as well as other ground cover, keeps the soil around the sclerotia moist for an extended period of time, at least long enough to allow them to germinate and produce the mushroom-like apothecia. Mowing the orchard floor, particularly in orchards with a history of calyx end rot, to keep vegetation short during the 10 to 14 days leading up to pedal fall, will allow soils to dry and reduce the environmental conditions required for sclerotia germination and consequent spore production.

Picking and removing infected fruit from the orchard during mid season thinning may also help in reducing inoculum from building up in the orchard. In the past, applications of a benzimidazole fungicide such as Benlate for scab control around bloom time protected senescent petals from infection and probably kept calyx end rot at bay. However, with the loss of Benlate for scab management in Ontario, apple growers may see a rise in this disease particularly in seasons following prolong wet and cool environmental conditions during blossom time.