Pests AND environmental PROBLEMS of High Altitude Landscapes

Jessica Hendryx B.S. in Horticulture Science with a Landscape Design Option ~ Montana State University, Bozeman

Black Knot  (Dibotryon)

Susceptible Plants to Black Knot: European Bird Cherry  (Prunus padus); Chokecherry  (Prunus virginiana)

Less Susceptible Plants:  Sour or Tart Cherries (Prunus sp.)

Alternate Plant Selection: Mountain Alder  (Alnus sinuata)

What is Black Knot?

Black knot  (Dibotryon morbosum) is a fungus that can be very destructive on the Prunus genus. It was first described in 1821 in Pennsylvania and in the 1880's  was considered the most destructive disease of tart cherries and plums. Today the disease is not as destructive, however it is still very common throughout the U.S.  It is prevalent in the southeastern U.S. where the fungus heavily infects cultivated and wild plums and cherries. In the north western U.S. the disease commonly infects many natives such as mayday tree, chokecherry, and native plum, and are not as severe on European cherries.  

Symptoms:

Black knot is a fungus that causes the cells within the plant to produce rapidly. The rapid production of cells create large irregular swellings on the branches. The most predominant disease identification is the irregular shaped black knots that only affect the woody parts of the plants, specifically twigs. These knots can swell 1.5 - 30cm in length and up to 7.5cm wide. The knots rarely swell around the entire limb, however numerous infections on branches can cause a decline in the plant health and in a few years, death. When the knots first form they are corky, olive-green, and firm. In time the knots turn black, hard, and brittle (See Figure A & B). The knots persist each year and expand lengthwise each year.

Overall: Look for the common symptom of black knots that make the plant appear grotesque. 

How is it Spread?

In the spring and early fall the fungus can spread by both asexual and sexual structures, known as conidia and ascospores, respectively. These structures help to aid the fungal spores in transport. However, it is moisture that is the key element in transport. The infection and transport must occur during wet periods, rainfall of 6 hours or more is ideal. The conidia and ascospores are spread by wind and rain and can inoculate both healthy and injured woody tissue of the current seasons growth. Most infection occurs just before and during flower blooming. In some climates a highly infectious time also includes just after leaf drop. However it is usually too dry in the western states for this to occur. The fungus then grows into the cambium, along the axis of the branch, and xylem parenchyma. After 5-6 months excessive parenchyma cells are produced and pushed outward forming the characteristic knot. If infected in the spring the first symptoms are usually visible by early fall. The second summer is when the knots begin developing rapidly.

 In the spring the conidia are produced on the surface of the knot which creates a white film over the black knot. During the second season of growth, the knot becomes significantly larger and perithecia, the fruiting bodies for the ascospores, are formed. The perithecia develop through the winter and in the spring the ascospores are released from these structures. The ascospores cause a greenish-olive colored appearance along the surface of the knot. During the spring it is more likely, however, to see the white film caused by the conidia because it persists longer than the greenish-olive colored appearance caused by the ascospores.   

What can you do about it?

If the disease has already infected the landscape plants it is possible to control the fungus by pruning. All pruning of the knots must be done in dry weather and pruned materials must be burned, buried, or bagged and disposed of properly. Prune 10 cm below the visible swelling and on large branches prune 2 cm beyond the swelling. Doing this during midsummer not only generally avoids the water issue, but it also is the period in which visible swelling is close to the advancing fungal growth in the stem. 

The fungus Dibotryon morbosum needs moisture to survive. Irrigation systems installed in landscapes do an excellent job keeping this fungus alive. Do not over water the plants and try to avoid watering at night. This will allow the water to stay on the plant for longer than it would during the heat of the day. If possible drip irrigate the landscape trees and shrubs. Doing this will avoid water on the twigs which will aid the disease development.

In the spring deer and elk will often chew on the new growth creating an opening for the fungus to enter. This is particularly a bad time because the moisture levels are higher during this period. This herbivore evasion is often a large problem in the landscape situation. There are many solutions that aid in detouring these animals from browsing, but infection will likely occur otherwise. More often than not these acts of prevention are  unsuccessful.

Overall, accept this disease as part of the habitat. Avoid planting susceptible plants such as European Bird Cherry - Prunus padus and Chokecherry - Prunus virginiana and understand that some plants may die because of this disease. If black knot is found in the area try substituting the Prunus plants with Mountain Alder - Alnus sinuata.

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References:

Hickey, K.D. Compendium of Stone Fruit Diseases. American Phythopathological Society Press: 1995 Pgs 34-35.

Agrios, G.N. Plant Pathology. Harcourt Academic Press: 1997 Pg 319.

Montana State University Extension Service 

Designed and researched by Jessica Hendryx - B.S. in Horticulture with a Landscape Design Option
For problems or questions regarding this web contact [martha@montana.edu].
Last updated: April 24, 2002.