Apple Tree Viruses

Virus and Other Graft-Transmissible Diseases and Disorders of Apples

Virus diseases cause economic losses through lower yields and reduced quality of apples and apple products. In general, virus diseases in perennial crop plants, such as apples, are more potentially damaging than in annual crops. Viruses can remain latent, spreading through an orchard and inflicting damage, often without the growers’ knowledge. Latent virus infection can produce small to moderate losses in fruit production. Often growers can maintain the productivity of diseased …

Apple Union Necrosis and Decline

Apple union necrosis and decline (AUND) is caused by tomato ringspot virus (TmRSV). TmRSV also causes disease in other fruit trees (Prunus stem pitting of peach and nectarine, brownline/constriction disease of Stanley prune) as well as in small fruits and many herbaceous plants. AUND is of economic importance in commercial apple orchards, where the virus most often is isolated from clonally propagated, size-controlling rootstocks. This disease is only a problem on grafted trees where the fruiting variety is resistant …

Bitter Rot of Apple

Bitter rot is a fungus disease that causes fruit rot during the summer and preharvest period. The bitter rot fungi are almost worldwide in distribution and cause an especially important economic disease in the southern areas of the United States. Outbreaks can occur rapidly, and losses can be severe, especially under prolonged warm, wet weather. Several host species can be affected. On peach and nectarine, the same fungus causes a disease known as anthracnose, and on grape it causes ripe …

Blister Spot of Apple

Blister spot is a bacterial disease of apple fruit caused by Pseudomonas syringae pv. papulans. This disease is of economic importance mainly on the cultivar ‘Mutsu’ (‘Crispin’) but can be seen on ‘Golden Delicious’ grown adjacent to ‘Mutsu’. Even though fruit grow to maturity and no detectable yield loss occurs, severe infection results in ugly fruit and greatly reduces fresh-market quality.

Figure 1. Blister spot on ‘Mutsu’ fruit. Photo: T. van der Zwet, USDA.
Blister spot on Mutsu (Crispin) fruit. Photo courtesy T. van der Zwet, USDA.

Blister spot lesions are first noticeable …

Black Pox of Apple

Black pox is a fungus disease caused by Helminthosporium papulosum. It is considered to be of minor economic importance. When it occurs, the disease can affect apple bark, fruit, and foliage. It is more common from southern Virginia southward than in the northern mid-Atlantic region. The same fungus causes blister canker on pear.

Infection first appears on current season twigs as well-defined, conical, shiny black lesions, which enlarge by the end of the first season. Extensive secondary lesion enlargement may …

Blue Mold on Apple

Blue mold is a fungus disease caused by Penicillium expansum. It is the most economically important post-harvest decay of stored apples in the United States. The losses from this disease can be significant but can be substantially reduced by following proper sanitation and control measures. The fungus not only causes fruit decay but also produces the carcinogenic mycotoxin patulin. This toxin may rise to unacceptable levels in fruit destined for processing and may result in off flavors.

Figure 1.

Brooks Spot on Apple

Figure 1. Brooks spot infection at about mid-season. Photo: Keith S. Yoder, Virginia Tech.
Brooks spot infection at about mid-season. Photo courtesy Keith S. Yoder, Virginia Tech.

Brooks spot is a fungus disease caused by Mycosphaerella pomi. It is a minor disease that occurs throughout the northeastern and mid-Atlantic apple growing regions of the United States and occasionally as far west as Iowa. The more susceptible common commercial cultivars include ‘Jonathan’, ‘Golden Delicious’, ‘Stayman’, ‘Grimes Golden’, and ‘Rome Beauty’.

Brooks spot first appears as irregular, slightly sunken dark green lesions typically on the …

Cedar-Apple Rust

Figure 1. Cedar-apple rust gall on eastern red cedar showing thoroughly wetted, gelatinous spore-bearing structures. (Alan R. Biggs, West Virginia University)

Cedar-apple rust is a fungus disease of apple and cedar and spends parts of its life cycle on each host. It is caused by Gymnosporangium juniperi-virginianae. The fungus can infect leaves and fruit of most cultivars in the eastern region. A notable exception is ‘Delicious’, which is nearly immune.

The most conspicuous symptoms on apple are bright orange, glistening …

Table of Apple Cultivar Susceptibility to Powdery Mildew

Powdery mildew on apple is a fungus disease caused by Podosphaera leucotricha. It can be a persistent disease of susceptible apple cultivars wherever apples are grown. It is the only fungal apple disease that is capable of infecting without wetting from rain or dew. Mildew severity and the need for control measures are related to cultivar susceptibility and intended fruit market. The table below lists the relative powdery mildew susceptibility of numerous apple cultivars. 

Apple cultivar Powdery mildew susceptibility rating