In This Section | Frog-Eye Leaf and Spot Black Rot
Frog-eye leaf spots caused by the fungus Botryosphaeria obtusa appears to be common in several apple orchards this year. The disease often first shows up 1-3 weeks after petal fall; however, this year the disease appears to be more common in some orchards than in previous years. On leaves the disease first appears as a tiny purple fleck, which eventually enlarges into a circular lesion about 4-5 mm in diameter (Figure 1). As the lesion enlarges, the margin remains purple while the center turns tan or brown giving the lesion a “frog-eye” appearance. The optimum temperature for leaf infections is around 26.6oC with 4.5 hours of leaf wetness. Fruit can also be infected by the fungus resulting in the disease known as black rot (Figure 2). Initial symptoms on young fruit appear as red flecks that develop into purplish, slightly raised pimples, which often are unnoticed at first. Infections of mature fruit appear as irregular black spots surrounded by red halos. Regardless, as the fruit begins to mature and ripen, the lesions begin to enlarge and produce concentric dark rings. Often a colonized mummified fruit is located in close proximity to the black rot infected fruit.
Figure 1. Frog-eye leaf spot symptoms on apple.
Figure 2. Advanced fruit symptoms on black rot on Royal Gala. Sepals of fruit can become infected early in the season just after bud scales become lose. Red specks on sepals are the first symptoms of early infection that turn purple surrounded by red rings. Early sepal infections eventually develop into blossom end rot. The fungus can also proceed into the core, which becomes rotten and results in premature fruit drop before external symptoms on the fruit are noticeable. Damage to limbs caused by wounds, cankers from other disease, insects or winter injury can provide an entrance for the fungus to invade and become established. Limb cankers first appear as reddish or pinkish, brown sunken areas in the bark (Figure 3). Cankers often remain small and superficial but can sometimes enlarge up to half a meter in length along the infected limb, killing the bark which eventually cracks.
Figure 3. Early Black rot canker on apple tree limb. One reason for the higher incidence of this disease in Ontario orchards over the past 2 year may be due to the winter injury on limbs sustained in some apple orchards during the 2004-5 winters, which provided the entrance wounds for the pathogen during the 2005 and 2006 seasons. Cankers that developed in orchards in 2005 provide the inoculum resulting in more mummified fruit last year in 2006. The mummified fruit left in trees is now providing inoculum for leaf infections resulting in the higher incidence of Frog-eyed leaf spot observed this time of year. Usually, a mummified fruit can be found within the vicinity of where the patches of frog-eye leaf spot symptoms are observed. Pruning out diseased limbs and dead wood is an important practice to reduce the inoculum sources within the orchard. Be sure to remove the prunings from the orchard or burn them, since the fungus can survive on dead tissue. Alternatively, chopping up the prunings on the orchard floor with a flail mower will also reduce inoculum levels without the hassle of removing them from the orchard. Pruning cuts can sometimes be categorized as wounds that won’t heal. This is because the cut is not made to wound periderm and has no possibility of healing. Always cut to wound periderm to prevent fungus from colonizining an unhealed pruning cut. Since, mummified fruit often left in trees become infected with the fungus and remain as a source of inoculum within the orchard, the removal of mummified fruit will also help reduce the potential of the inoculum building up within the orchard that can infect leaves and fruit. Fungicide sprays with Captan starting at silver tip on a 10 to 14 day schedule will protect leaves from getting the frogeye leaf spot as well as the fruit from becoming infected and developing black rot.
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