Lexington, Kentucky 40546
Nursery Update - A University of Kentucky Cooperative Extension Service update for the Kentucky Nursery Industry
By Amy Fulcher, Extension Associate - Nursery Crops
University of Kentucky Department of Horticulture
Update #11
April 23, 2003

Diseases of Crabapples
Spring weather means great conditions for fireblight, rust, and apple scab.

Scab (primary infection): At an average temperature of 60F and 13 hours of leaf wetness a moderate infection can take place.

Rust: Six to seven hours of leaf wetness when the temperature is in the 50s-60s can cause moderate infections. Danger of infection usually ends at 30 days after bloom. At this point the fungus stops producing spores and the foliage is less succulent.

Fireblight: Infection is favored by humid weather with 65-70F degree temperatures during and after bloom and occasional rains. Stormy weather, especially hail, after primary infections can mean significant secondary infections.

See U.K. Pub. Woody Plant Disease Control Guide for Kentucky ID-88 or the IPM Calendar for Deciduous Tree Production for spray recommendations.

Sources: Apple Scab. U.K. Pub. PPA-24, Rust Diseases of Apple. U.K. Pub. PPA-23, Fireblight U.K. Pub. PPA-34.

Fireblight canker - cracked edge and sunken, purplish interior. Remove suckers; their lush growth prolongs fireblight infection and allows the infection to go straight to the base of the trunk.


Crown Gall

Crown gall infects many species of plants, usually forming at wound sites. The gall restricts the flow of nutrients, slowing plant growth. The appearance of a gall can make the plant unsalable. Infected plants should be rogued and destroyed.

Source: Diseases of Woody Ornamentals and Trees in Nurseries. Jones and Benson, eds. APS Press


Lilac Borer
Male lilac borers were caught in traps with pheromone lures in Graves County last week.

Lilac borer, also known as the ash borer, is a severe pest of ash, lilac, and privet. Larvae cause damage by making tunnels in the bark and wood of trunks and branches. Most infections are from the root crown to 3'. Females lay eggs in bark crevices or ridges almost always near wounds. After hatching, larvae chew into the bark and feed on the phloem. Later the larvae concentrate feeding on the sapwood, disrupting water conduction and causing dieback. Early symptoms include sap and sawdust-like frass at the entrance holes.

Avoid pruning when the adult stage is present. Spray 10-14 days after the first males are caught which will coincide with the larval hatching period. If using phenology to time sprays, apply Astro or Dursban 1 week after full bloom of flowering dogwood.

Source: Insect Borers of Trees and Shrubs. U.K. Pub. ENT-43, Insects That Feed on Trees and Shrubs. Johnson and Lyon. 1991. Cornell University Press.

Cold Damage

Cold damage can take several forms. Left: The lower trunk is the last part of the tree to harden for winter. Excess fertilizer, late fertilization, exposed overwintering site, or drying out can predispose a tree to cold injury. Center: The blackened area above and below the node was likely damaged because vigorous growth on the branches from that node prevented that section of trunk from completely hardening. This phenomenon is seen when witches brooms on ash yellows-infected trees prevent hardening. Right: The holes resulted from cold, but not severe, temperatures when the tree was beginning to bud out. The individual cells died. As the leaf expands the open areas where cells were killed will become larger.

Source: Diseases of Trees and Shrubs. Sinclair, Lyon, and Johnson. Cornell University Press. Preparing Nursery Plants for Winter. NC Pub. AG-454.


Educational programs of the Kentucky Cooperative Extension Service serve all people regardless of race, color, age, sex, religion, disability, or national origin.

Note: Trade names are used to simplify the information presented. No endorsement by the Kentucky Cooperative Extension Service is intended, nor is criticism implied of similar products that are not named. Always read product label before use.