apple disease – Page 2

August 22, 2019August 22, 2019

Table of Apple Cultivar Fire Blight Susceptibility

Fire blight is a destructive bacterial disease of apples and pears that kills blossoms, shoots, limbs, and, sometimes, entire trees. The disease is generally common throughout the United States wherever apples are grown. However, outbreaks are typically very erratic, causing severe losses in some orchards in some years and little or no significant damage in others. This table shows the fire blight resistance rating of numerous apple cultivars.


Apple cultivar	Fire blight resistance rating (z)	Apple cultivar	Fire blight resistance

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August 22, 2019August 22, 2019

'Golden Delicious' Necrotic Leaf Blotch in Apple Trees

Figure 1. ‘Golden Delicious’ apple leaves showing leaf blotch and yellowing associated with NLB. Photo: T. B. Sutton, North Carolina State University, and A. L. Jones, Michigan State University.

Necrotic leaf blotch (NLB) is a physiological disorder whose occurrence is related to air temperature, light intensity, and soil moisture. The cause of NLB of apple is not known. Symptoms that are enhanced by gibberellins and reduced by abscisic acid are evidence that a hormonal imbalance may be involved. The disorder …

August 22, 2019August 22, 2019

Nectria Twig Blight of Apple

Nectria twig blight is a fungus disease that results in dieback of apple twigs. It is caused by the fungus Nectria cinnabarina.

Recognition of the disease is important because the disease is often confused with fire blight, which requires different control measures. In late May to early June, shoot growth on infected twigs begins to wilt and die (fig. 1). Small, sunken cankers are found at the bases of the wilted shoots. Leaves on infected shoots appear to die from …

August 22, 2019August 22, 2019

Powdery Mildew on Apple

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. (Note: Many other powdery species occur on a wide variety of plant hosts.)

Powdery mildew causes whitish lesions on …

August 22, 2019August 22, 2019

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

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August 22, 2019August 22, 2019

Nectria Canker of Apple

Nectria canker is a fungus disease caused by Nectria galligena. Occasionally, it is found on apple nursery stock shipped into the eastern United States; however, the economic effects of the disease in the region are minor.

Figure 1. Nectria cankers on trunk of ‘Delicious’ apple. Photo: Alan L. Jones, Michigan State University.

The fungus grows deep into the wood and kills new wound callus as it develops. This annual killing of successive layers of callus results in perennial, target-like, zonate …

August 22, 2019August 22, 2019

Sooty Blotch and Flyspeck in Apple

Sooty blotch and flyspeck are surface blemish diseases caused by fungi that commonly appear together on apple or pear in late summer and fall. Although these diseases may shorten the storage life of fruit due to increased water loss, they do not cause decay, and losses are attributed to unacceptable appearance. During wet growing seasons, losses of 25% or higher commonly occur, even in orchards treated with fungicides. Many fungi are involved in this complex of pathogens, including Peltaster fructicola, …

August 22, 2019August 22, 2019

Burr Knots on Apple Trees

Burr knots are root initials that appear on the aboveground portion of the apple tree trunk. Some rootstocks, such as M.7, M.9, M.26, MM.106, and MM.111, are more prone to developing burr knots than others. Conditions that favor burr knot development include low light, high humidity, and temperatures between 68°F and 95°F.

Burr knots are problematic in a couple of ways. They can be an entry point for organisms, such as dogwood and plum borers, woolly apple aphids, fire blight …

August 22, 2019August 22, 2019

Phytophthora Root, Crown, and Collar Rot of Apple

Crown or collar rot has caused extensive death of apple trees in many eastern orchards during the past two decades. It often occurs on trees between 3 and 8 years of age grown on Malling-Merton 104 (MM. 104), MM. 106, and, to a lesser degree, MM. 111 rootstocks. The disease often affects low areas of orchards having heavy, poorly drained soils, but it can affect all orchard sites if trees are first infected in the nursery.

The first symptoms to …

August 22, 2019August 22, 2019

Table of Apple Rootstock Susceptibility to Phytophthora spp.

Apple Phytophthora root, crown, or collar rot has caused extensive death of apple trees in many eastern orchards during the past two decades. It often occurs on trees between 3 and 8 years of age grown on Malling-Merton 104 (MM.104), MM.106, and, to a lesser degree, MM.111 rootstocks. The disease often affects low areas of orchards having heavy, poorly drained soils, but it can affect all orchard sites if trees are first infected in the nursery. This table illustrates susceptibility …