White rot is often referred to as Botryosphaeria rot or Bot rot and is caused by Botryosphaeria dothidea, a serious fungal pathogen of apple wood and fruit. The fungus causing white rot is ubiquitous in nature, occurring on a wide variety of woody plants, including birch, chestnut, peach, and blueberry. The canker phase of the pathogen can cause considerable loss in many parts of the South, Midwest, and Northeast. Fruit rot infection is most common in areas of the southeastern United States, where losses of up to 50% have been reported. An increase in infection and canker expansion can occur under conditions of drought stress and winter injury.
The white rot fungus infects only wood and fruit.
- New infections on twigs and limbs become evident in early summer and originate around lenticels, appearing as small, circular spots or blisters. As the lesions expand, the area becomes depressed, and a watery exudate may appear on the bark around the blisters. In 4 to 8 weeks, black fruiting structures grow within cankers. Cankers stop enlarging by late fall and are indistinguishable from black rot cankers (caused by B. obtusa), making isolation of the pathogen necessary for correct identification of the causal organism. As the cankers progress, they exhibit a scaly, papery, orange outer bark that often sloughs off (fig. 1). Under favorable conditions for disease development, cankers fuse, and the girdling of large limbs can occur. The sudden appearance of bright yellow foliage on some apple limbs in late May to early June—one of the more striking symptoms of white rot—occurs when a canker associated with a wound girdles the limb.
- Symptoms of fruit rot infection can appear 4 to 6 weeks before harvest, depending on the developmental stage of the fruit. Lesions begin as small, slightly sunken brown spots that may be surrounded by a red halo. As the decayed area expands, the core of the fruit becomes rotten, and eventually the entire fruit rots. Black fruiting structures may grow on the surface of the rotted fruit in advanced stages. Red-skinned apple cultivars may “bleach” during the decay process, becoming light brown. The disease is referred to as white rot because of this characteristic. Decayed flesh associated with white rot is very soft and watery under warm conditions (fig. 2). When fruit rot develops under cool conditions, however, the rotted area is much firmer and is similar to black rot infection. On certain apple cultivars, even with these criteria, determining which fungus initiated the infection is difficult. Rotted fruit usually drop from the tree, but some mummify and remain attached.
Figure 1. Canker on an apple limb caused by the white rot fungus. Photo: James W. Travis, Penn State University.
|Figure 2. ‘Golden Delicious’ fruit infected by the white rot fungus. Photo: James W. Travis, Penn State University.|
The disease cycle for white rot is almost identical to that of black rot. The host range of white rot is broad, but the role that hosts other than apple play in the development of the disease in apples is not known. The white rot fungus overwinters in cankered wood, wood previously killed by fire blight, dead bark, and mummified fruit. Ascospores and conidia develop on these structures throughout the growing season. The optimum temperature for germination of both spore types is 82°F to 90°F, with germination occurring in as little as 90 minutes at 82°F. During wet periods, spores ooze out of fruiting structures and are dispersed by rainfall. Infection of wood can occur through lenticels and wounds. Also, moisture stress (drought) and winter injury facilitate canker development, especially on older limbs.
It is not known exactly when fruit infection occurs. Some theories propose that infection can occur anytime from the bloom period to harvest. Other theories state that fruit infection occurs only during the last 6 to 8 weeks of the growing season and that the degree of infection is dependent on the sugar content of fruit of individual cultivars. Wounding is not necessary for fruit infection to occur; however, when wounds are present, the fungus colonizes them rapidly. Infection of wounded fruit can occur in as little as 2 hours at 82°F; however, 16 hours of a wetting period is necessary for infection to occur at 46°F.
You may employ both cultural and chemical control practices to minimize white rot. Prune and remove all dead wood, including spurs, twigs, and branches, because the fungus survives in these structures. Also, remove the current season’s fire blighted shoots, which can be colonized rapidly by B. dothidea. Irrigate trees during periods of hot, dry weather to minimize drought stress, which predisposes the tree to twig and branch infections. Cultivars do not vary greatly in their susceptibility to white rot, although ‘Golden Delicious’, ‘Empire’, and ‘Jersey Mac’ appear more severely affected than others. Use a fungicide spray program from bloom through harvest to protect against white rot infections.
Original text prepared by J. W. Travis, J. L. Rytter, and A. R. Biggs. The original version of this article appeared in The Mid-Atlantic Orchard Monitoring Guide (NRAES-75) and is reproduced with permission from the Natural Resource, Agriculture, and Engineering Service, Cooperative Extension, 152 Riley-Robb Hall, Ithaca, New York 14853-5701, U.S.A. (607) 255-7654. It has been edited for presentation here by Alan R. Biggs, West Virginia University.