Overview:

Black knot is a cankering fungus disease that infects many trees and shrubs within the genus Prunus. It is caused by the native North American fungal pathogen Apiosporina morbosa. Although black knot fungal galls have been found in a couple of dozen Prunus species, not all species are equally susceptible, and it can be most destructive to certain varieties of cherries and plums (Photo 1).

The following species can be highly susceptible to infections: Japanese plums (Prunus salicina), European and American plums (Prunus domestica & Prunus americana), Sweet cherry (Prunus avium), Mahaleb cherry (Prunus mahaleb), and cultivated species of chokecherries.

This blog reviews the symptoms and signs of Black Knot Disease. It’s management by pruning and fungicide treatments is discussed. Finally, a handful of resistant plum varieties is provided.

Photo 1: This Japanese Plum species is highly susceptible to Black Knot Disease. (Photo Credit: Steven K. Rettke, Rutgers Coop. Ext.)

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Curiosity of Plant Galls:

Explaining the unusual gall growths or deformities in plants can be challenging. Different galls can be found on virtually any plant part, including leaves, flowers, twigs, bark, or buds of trees and shrubs. Galls caused by insects or mites can typically be identified by the characteristic size, shape, color, and structure that develops.

Insect/mite galls are produced by the interaction of plant hormones with strong plant-regulating chemicals released by the gall maker as they feed. The immature larvae live their entire lives within an individual gall, which provides excellent protection for the larvae as they feed and mature.

This blog contains information on horned and gouty oak galls. When the number of galls infesting an oak is few, tree damage is insignificant. However, tree injuries become more severe when gall numbers reach high levels.   

Curious & colorful leaf galls created by maple bladder galls mites may cause alarm to some people but they are mostly harmless. (Photo Credit: Steven K. Rettke, Rutgers Coop. Ext.)

An extreme infestation of horned oak galls on branches of pin oak tree. (Photo Credit: Steven K. Rettke, Rutgers Coop. Ext.)

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Dormant Oil Guidelines:

Dormant oils at 2-4% rates in the late fall & late winter seasons can aid in the control of overwintering insects and insect eggs. Consider treating for such pests as aphids (eggs), southern red mites/eggs, spruce spider mites/eggs, oak spider mites (eggs), eriophyid mites/eggs, spruce gall adelgids, lace bugs (deciduous plants), cankerworms (eggs), leaftiers, psyllids, plant bugs, etc.

Some landscapers have been successful applying both late fall and early spring dormant oil treatments to the same plants on the same properties. This is a reasonable approach since a single dormant oil treatment will not provide 100% control and it gives the field technician another opportunity to monitor the landscape for problems.

Dormant oil sprays require excellent coverage & often repeat applications. (Photo Credit: Steven K. Rettke, Rutgers Coop. Ext.)

Japanese maple scale infested branch will likely require multiple dormant oil treatments to suppress. (Photo Credit: Steven K. Rettke, Rutgers Coop. Ext.)

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General Overview:

There are 30 non-native ambrosia beetle species in North America but only a half dozen are serious insect pests within nurseries, pecans & apples. The Xylosandrus species of beetle borers are native to southeast Asia & were first reported in the United States during the early 1930s (black stem borer). Today they are found in 29 states. Two ambrosia species found in NJ nurseries cause serious damage to trees. (1) The black stem borer (Xylosandrus germanus) (~2 mm) has attacked more than 200 tree species (fig.1). (2) The granulate (Asian) borer (Xylosandrus crassiusculus) (~2.5 mm) has attacked more than 120 tree species (fig.2). Both species are a threat to trees that are stressed. A 3rd species, the invasive redbay ambrosia beetle (Xyleborus glabratus), was first found in Louisiana in 2014. A major difference is it will also attack trees not under stress. It is not presently found in NJ, but it is moving north. The concern is the impact it may have on sassafras forests.

Fig. 1: An adult female ambrosia black stem beetle borer. Photo Credit: Brad Barnd, Bug Guide.net

Fig 2: An adult female ambrosia granulate borer. Photo Credit: Alabama Coop. Ext.

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