Michael Celetti, OMAFRA Plant Pathologists – Horticulture Crops
Several apple growers and consultants have noticed developing apples fruitlet with petals remaining attached. These delicate petals can be a source of tissue and avenue for infection by Sclerotinia sclerotiorum which causes the disease calyx end rot. The pathogen causes the disease ‘White mold’ in many crops such as carrots, lettuce, beans, canola, potatoes, tomatoes and has a very large host range including many common weeds such as dandelion.
The pathogen produces a persistent resting structure called a ‘sclerotia’ that can last in soil for many years. Sclerotia that are near or at the surface of soil in orchards germinate under wet conditions to produce tiny funnel shaped mushroom-like structures called ‘apothecia’. The tiny apothecia produce and ejected spores that infect the delicate senescent flower petals. Fortunately the spores cannot infect the fruit directly. Once the flower petals become infected and colonized, the fungus grows from the infected petals into the developing fruit. Therefore fruitlets with petals still attached are more vulnerable to infection.
Figure 1. Symptoms of calyx end rot on a developing apple fruit
The best time to scout for symptoms of this disease is in mid–late June or early July. Calyx end rot first appears as a slightly sunken circular tan brown to grey lesion around the calyx of developing fruit. The lesions are dry for most of the season unless secondary pathogens such as bacteria enter through the wound. As the lesion expands they tend to grow in one direction, appearing slightly off centre from the calyx and may be surrounded by a dark border and/or a red halo (Figure 1). Symptoms of calyx end rot can be confused with dry eye rot caused by Botrytis cineria, which tends to occur later in the season, or sometimes with black rot infections at the calyx end of the fruit. Infected fruit eventually crack and the hard black sclerotia develop in the cracks. After the fruit rots, the sclerotia are released where they can remain in or on the soil of the orchard floor for several years.
Successful disease management depends on the weather and reducing alternative hosts and manipulating the environmental conditions that favour sclerotia germination. It takes several days of wet soil conditions and temperatures above 11-15oC for sclerotia to germinate and produce apothecia. If soil conditions are too dry, the sclerotia will not germinated but remain dormant until conditions become conducive for germination. In other susceptible crops such as carrots, researchers discovered that trimming the carrot leaves to open up the canopy and allow the soil conditions to dry out quickly after a rain, resulted in significantly fewer sclerotia germinating, less spores being produced and significantly less disease. In fact, the research has led to the development of new equipment designed specifically for trimming carrot leaves between rows to open the canopy and reduce sclerotia germination. Trimming leaves at row closure is now a standard recommendation for white mold management in carrots. The same principle can be applied to apple orchards with tall grass or even worse, dandelion infested lane ways. Keeping the grass and particularly dandelions cut short will allow the soil to dry quickly and reduce the environmental conditions required for sclerotia to germination, resulting in less infection of petals. Since dandelions and other broad leaf weeds are hosts of this pathogen, good broadleaf weed control in the orchard will also prevent the build up of sclerotia population in the orchard.
There are no fungicides registered in Ontario specifically for the control of calyx end rot on apples, however, some of the new group 7 fungicides available for powdery mildew and scab control may also protect petals from Sclerotinia infection. As with any fungicide, application must occur prior to infection for best results. Infected fruit should be hand thinned and removed from the orchard since throwing infected fruit onto the orchard floor will result in more inoculum for future years.