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 …
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 …
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 …
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. This table lists juniper, hawthorn, and crab apple cultivars with resistance to rust diseases.
| Juniperus (Juniper) | Crataegus (Hawthorn) | Malus (Crab Apple) |
|---|---|---|
| Cedar-apple and hawthorn rust resistant | Cedar hawthorn rust resistant | Cedar hawthorn rust resistant |
| J. ashei | C. crus-galli | ‘Adams’ |
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 |
|---|
| 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 …
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 …
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 …
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 |
|---|
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 …
