Wet winter plaguing plants

Published 9:47 am Wednesday, March 28, 2012

By Neil Clark and Janet Spencer

For plants, water is an essential element for vigorous growth. It can also be a rapid vector for agents of harm – fungi.

Rain has been a frequent visitor here since last August. One of the continuing after effects of hurricane Irene is broken branches and created scars on trees and shrubs by whipping them around.

The storm also likely transported many and varied fungal spores great distances across the landscape. It seems there are hundreds of examples of fungal pathogens that typically live in the soil waiting for wet weather to create the prime conditions for their reproduction and dispersal.

These spores then find an entry point into the plant either through places where insects have pierced leaves or twigs, or sometimes through cracks in the bark. These fungi then go on to “clog the arteries” (the xylem and phloem . . . remember your fourth-grade biology class?) preventing the transport of food and nutrients throughout the plant, ultimately resulting in the plant dying.

Our office has received many calls this year withfolks who have had red tip with dying leaves and branches. This is being caused by what is known as leaf spot. This fungus also affects India hawthorn, some pear cultivars, as well as many other woody ornamentals in the rose family.

Leaf spot wreaks the most havoc during periods of cool, wet weather and when active growth is occurring.

On red tip plants, spots about the size of a pencil eraser have ashen gray centers and dark purple edges, which fade into a lighter purplish halo. The centers of the dead spots are dotted with minute black specks, which are the spore-producing structures. On Indian hawthorn, these blotches are bright red, and on pears, exhibit a thin brown edge.

These spots eventually merge to form large, irregular blotches, then entire leaves die. Because the fungus preferentially infects tender, new growth, cultural practices that stimulate succulent growth, such as summer pruning or frequent pruning or fertilization, favor disease.

As with most fungal diseases, prevention is key. In the case of red tip, avoid summer pruning (of course avoiding hurricanes pruning for you cannot be helped) and summer fertilization that favors succulent growth.

Fungicides, such as thiophanate methyl or myclobutanil, can be used preventatively, but they must be applied at the first sign of disease or when new growth starts. Repeat application every 7 to 14 days. Be sure to follow the directions on the label.

Fungicides should not be applied during hot, dry periods. If disease pressure is severe, substitutes such as camellia, fringe-flower, Carolina cherry laurel or wax myrtle may be better choices over the long term.

Unfortunately, the specialists at Virginia Tech send me updates monthly on new pathogens on the horizon affecting boxwoods, black walnut and the list goes on. Monitoring and early detection can help in many instances where fungus enters through leaves or branch tips and there’s a chance that pruning may remove this before it gets too far along.

But unfortunately, often these fungi enter further down the trunk, or the plant is so inundated that pruning is of little use.

Pruning calendars and plant care information are available at our offices as well as latest pest treatment recommendations.

Neil Clark is the southeast regional forestry extension agent and the unit coordinator with Virginia Cooperative Extenstion.
Janet Spencer is the agricultural extension agent and unit coordinator with Virginia Cooperative Extension in Isle of Wight.