Cedar apple rust is a common disease of apple trees that causes premature defoliation of apple trees, crop loss, or fruit lesions on susceptible cultivars. This disease requires two host plants to complete its life cycle, which are apple and eastern red cedar (Juniperus virginiana). However, other Juniperus species can also serve as an alternate host for cedar apple rust.
Apple cultivars vary in their susceptibility to cedar apple rust disease. Cultivars such as Delicious, Enterprise, Golden Supreme, Liberty, McIntosh, and Priscilla are resistant to the disease. However, Braeburn, Cameo, Jonathan, Golden Delicious, GoldRush, Prima, Rome Beauty, York Imperial are highly susceptible to cedar apple rust and can be severely damaged when infection occurs.
Cedar apple rust infection on apple occurs in the spring when apple flower buds are visible, but still closed (i.e., tight cluster stage), until after petal fall. Also, four to eight day- old leaves just emerging from buds are very susceptible to apple cedar rust infection. Symptoms of this disease appear as yellow-orange lesions on the upper surface of leaves, petioles, or developing fruit. Later, a red-orange halo surrounds each lesion on the foliage of susceptible apple cultivars (Figure 1). After several weeks, yellow-brown lesions with tubular protrusions develop on the underside of leaves or on the surface of developing apples (Figure 2). Within these lesions, structures called aecia produce aeciospores.
In late summer, aeciospores are released from the aecia and subsequently infect leaves of eastern red cedar and form galls (Figure 3). The following spring, the galls produce orange structures called telia. When it rains, these horn-like telia swell and become gelatinous (Figure 4), but after drying, they shrink back to dark brown threads. With frequent rainfall, swelling and shrinking of telia may occur as many as ten times during the spring. During rainy periods, spores produced by the telia germinate and other types of spores, called basidiospores, are produced. Basidiospores are subsequently discharged into the air and can travel more than a half mile. When these spores land on apple leaves or floral tissues, and environmental conditions are favorable, cedar apple rust infection occurs. The optimum conditions for infection of apple are when plant surfaces are wet for four or more hours and temperatures range from 52 to 77°F. However, infection can also occur at temperatures as low as 77°F when rainfall occurs for seven hours.
To prevent cedar apple rust, plant resistant apple cultivars. Although removal of eastern red cedar growing near rust susceptible apple trees can reduce rust infection, the disease is not always eliminated, since disease spores can travel long distances on air currents. For apple cultivars susceptible to rust, fungicides can be applied from tight cluster flower stage to about 30 days after petal fall. For home fruit production, Captan or Captan + Mancozeb fungicides may be applied. Fungicide applications for commercial producers can be found at: https://ag.purdue.edu/hla/Hort/Documents/ID-465.pdf.
REVISED: February 21, 2017