Species of Malus (apple and crabapple) and Pyrus (pear) can be hosts, but there is variation in susceptibility among species and varieties. The pathogen can also cause peach rusty spot on fruit of Prunus domestica (peach) .
The pathogen is Podosphaera leucotricha, in a group of Ascomycetes called powdery mildews (Erysiphales: Erysiphaceae). This is one of the unusual cases where the group of pathogens have a common name and it is the same as the name given to the type of diseases they cause.
Powdery mildews are obligate parasites whose mycelium is completely superficial except for haustoria that penetrate the epidermal cells of the host. Haustoria take nutrients and water from the host cells.
The genus Podosphaera is easy to recognize microscopically (when it is fruiting sexually) because the cleistothecia (closed, globose ascomata; now often called chasmothecia for this group only) are large, black, contain a single ascus, and have large appendages, sometimes with dichotomously branched tips (see last image). The specific epithet “leucotricha” means white hairs, but the appendages do blacken from the base.
Like most powdery mildews, P. leucotricha also produces conidia in chains on the mycelium. These are produced in large numbers throughout the growing season.
Despite the fact that they are completely exposed on the plant surface, powdery mildews are an exception to the rule that free moisture on the plant surface is required by foliage pathogens for sporulation and infection. In fact, spore dispersal is inhibited when leaves are wet.
The fungus overwinters as mycelium in infected buds . This is a great strategy, because the fungus can continually infect all leaves and shoots developed from that bud in spring and summer.
Conidia are wind-dispersed to initiate secondary infections, usually on the undersides of leaves. These cycles of infection can continue. Under ideal conditions, this can result in logarithmic growth of numbers of infections until the amount of healthy susceptible tissue becomes limiting.
In summer, mycelium may darken and cleistothecia begin to form. The wind-dispersed ascospores are not as important as conidia in creating new infections, but undoubtedly are important in promoting adaptation to new conditions and host genotypes.
As winter buds are formed, P. leucotricha invades them and prepares for a long winter’s nap.
Distribution and Damage
This pathogen occurs around the world wherever apples and pears are grown . Infected flower buds produce malformed, discolored flowers. Leaves are not killed outright, but foliage on heavily infected trees is unsightly. Infected buds are more susceptible than healthy buds to being killed by winter injury, which damages the tree but also reduces primary inoculum for the coming year.
The best way to manage this disease is through selection of resistant varieties. However, the disease is not so frequently damaging that it is a primary consideration in variety selection.
Primary inoculum can be reduced by pruning infected, overwintering flower and shoot buds during winter or early spring . Infected twigs can be recognized in winter by their silvery gray color and glistening appearance in the sunlight, their stunted growth, and the reddish color and slender, elongated shape of their lateral buds . Pruning of whitened terminal shoots during the growing season can also help. It removes inoculum in the current year, but also prevents overwintering of the fungus in the infected shoot, reducing primary inoculum the following spring . However, if infection is severe enough to justify the labor and time of pruning infected twigs, it would likely mean pruning away much of the tree. Reducing inoculum by pruning is difficult in a large tree, and in a small tree it could conflict with development of desired tree architecture. Also, infected winter buds are easily missed.
Chemical control is often needed for commercial apple orchards, but may not be warranted in amenity plantings. Chemical control specific to flowering pear is discussed in a Pacific Northwest Extension web page .
- 1.Fisher DF. 1918. Apple Powdery Mildew and its Control in the Arid Regions of the Pacific Northwest. Bulletin No. 712. Washington, D.C.: United States Department of Agriculture. 28 pp. <https://archive.org/details/bulletinofusdep701725unit/page/256>.
- 2.Jankovics T, Dolovac N, Bulajić A, Krstić B, Pascal T, Bardin M, Nicot PC, Kiss L. 2011. Peach rusty spot Is caused by the apple powdery mildew fungus, Podosphaera leucotricha. Plant Disease, pp. 719–724 <10.1094/pdis-10-10-0711>.
- 3.Marine SC, Yoder KS, Baudoin A. 2010. Powdery Mildew of Apple. Plant Health Instructor <10.1094/PHI-I-2010-1021-01>.
- 4.Pacific Northwest Extension. 2015. Pear (Pyrus spp.)-Powdery Mildew. Pacific Northwest Pest Management Handbooks. <https://pnwhandbooks.org/plantdisease/host-disease/pear-pyrus-spp-powdery-mildew>.