Roanoke Times
Copyright (c) 1995, Landmark Communications, Inc.
DATE: THURSDAY, June 24, 1993 TAG: 9306240174
SECTION: CURRENT PAGE: NRV-4 EDITION: NEW RIVER VALLEY
SOURCE: Joe Hunnings
DATELINE: LENGTH: Long
Fungi, bacteria and other microorganisms that attach to plants also are flourishing.
Symptoms of infection by many of these organisms are evident now. However, for most of these diseases it is too late to control the problem. It is a good time to become familiar with these maladies so you can be prepared for any necessary control that may be needed next year.
Let's look at a few diseases I have seen in the New River Valley recently.
Anthracnose diseases
"Anthracnose" is a term plant pathologists use to describe a group of diseases that are caused by related species of fungi. Each anthracnose pathogen has a unique host range, so the fungus that causes sycamore anthracnose, for example, is not the same species as the one that causes dogwood anthracnose. Some of the anthracnose diseases we see in spring are:
\ Spot anthracnose of dogwood: This particular anthracnose disease of dogwood is not to be confused with the lethal discula anthracnose disease of dogwood. Spot anthracnose does not cause dieback, but it does cause spotting of leaves and flower bracts in spring. Symptoms of this disease are easy to recognize: spots on both bracts and leaves are tiny (generally the size of a pinhead) and uniform in size. Considerable browning of bracts may occur in wet springs, and infected mature leaves may appear distorted because of spotting early in development.
Fungicides such a Daconil 2787 applied when buds begin to open, when bracts have fallen, four weeks after bracts fall and again in late summer will control the disease.
\ Discula anthracnose of dogwood: This disease is the dreaded anthracnose that can kill entire trees within a relatively short time of infection. The pathogen infects bracts in the spring, causing dark brown spots irregular in size and shape. When leaves emerge, they, too, develop irregular spots with purplish or dark brown borders and lighter brown centers. Spots may coalesce and blight entire leaves. The pathogen does not stop at the leaves; it moves into twigs and ultimately into branches or the trunk, causing cankers from which the tree cannot recover. Branches tend to die from the bottom of the tree up, hence the earlier name for this disease: "lower branch dieback". Infected leaves may cling to twigs over the winter. This disease is especially a problem at high elevations, near water sources and under conditions of cool temperatures, high moisture, and shade.
Foliar protectant fungicides such as Daconil 2787 can be used to prevent the foliar phase of the disease; however, better control has been reported with the use of Banner, a systemic fungicide. Fungicides are not effective once cankers have formed. Cankered branches should be pruned out.
Peach leaf curl
A fungus disease found throughout the world where peaches are grown, peach leaf curl is carried over winter by tiny fungus spores lodged on the surfaces of twigs and buds of the peach or nectarine trees. With the coming of spring and the swelling of buds, if conditions of moisture and temperature are suitable, the spores germinate, and those that come into contact with the young developing leaves cause an infection. The infected leaves are thickened, and, as they develop, the leaf becomes folded with edges curling inward, so that the undersurface of the leaf is a series of concaved chambers. Very shortly after leaf symptoms appear, it turns red to purple and becomes extremely conspicuous. The bright color soon fades into a yellowish-brown to brown, and the leaf withers and falls off. One application of Ferbam, lime sulfur or bordeaux mixture during November or early spring before bud break will control this disease.
Black knot of prunus species
The key symptom of this disease, which occurs on many different cultivated and wild species of cherries and plums, is an elongated gall primarily on one side of the branch. Galls that are apparent this spring are actually the result of infections that occurred last spring. The surface of the gall is at first corky. It darkens during the growing season and turns hard and black the following winter. The twig may die above the gall or the gall may enlarge at the margins annually. Insects and secondary fungi may inhabit older knots.
To control the disease in ornamental species, remove galls by cutting ten centimeters beyond the swelling before spore dispersal begins in early spring and remove pruned material.
Use of fungicides such as a mixture of Benlate and Captan should also be applied prebloom and repeated two to three times at weekly intervals.
Fire blight of rosaceous plants
This bacterial disease affects many different plants in the rose family. Of the woody landscape plants, it is most conspicuous on crabapple, ornamental pear and flowering quince. It is also very common on fruiting apples and pears. The pathogen infects flowers in early spring and then moves into leaves and/or twigs. Symptoms are often not obvious until long after the initial infection. The most conspicuous symptom is a scorching of leaves. On crabapple, affected leaves are brown; on pear species, leaves are usually black, hence the name "fire blight." Wood on affected twigs is often cankered and bark appears darkened or cracked. Dieback occurs above cankers, and later in summer the tree may appear to have randomly scattered dead branches.
Control involves judicious pruning and destruction of affected branches. It is not a good idea to prune in springtime, as this may only result in further spread of the bacteria. The best time to prune is later in summer when bacteria go dormant. In any case, pruning tools should be surface-disinfected with ethanol or a dilute solution of bleach between cuts to avoid spreading bacteria.
Streptomycin sulfate, an antibiotic, is the most effective chemical control for fire blight. Apply just before center blossoms open for two or three sprays five days apart.
The most desirable control method is to select plant cultivars with resistance to fire blight. Ask your extension agent or nurseryman for recommendations.>
Apple rusts
Both cedar rust and quince rust are serious apple diseases in the Appalachian area. Red cedar is the alternate host for both the cedar-apple and quince rusts and severity of these diseases is related to the distance of the apple tree from infected cedars in the area. Cedar rust, caused by a fungus, appears as orange or greenish-yellow spots on the fruit and as yellowish to orange spots on the leaves. Leaf infection results in extensive defoliation and devitalization of the tree during dry periods. Cedar-apple rust galls, or "cedar apples," are located on the twigs of cedar. They develop masses of gelatinous spore horns during rainy periods, early in the growing season, from which spores are discharged that infect the apple.
Apple quince rust appears as sunken or deformed areas in the fruit, ranging from deep green to brown. Protectant sprays are required from early pink through June 10 for control of the rust diseases. Ferbam is the only fungicide listed here that adequately protects against rusts.
Joe Hunnings is the Virginia Cooperative Extension Service agent for agriculture in Christiansburg. If you have questions call the Montgomery County extension office at 382-5790.
by CNB