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Grape Disease Management

by Staff Writer - B. Shaughnessy | January 29, 2012

The proper management of a vineyard includes caring for grapes so that an excellent crop may be harvested. Caring for a vineyard includes monitoring rainfall to make sure that the grapes get enough water as well as eliminating threats from pests and wildlife. In addition to these concerns, the vintner must take a proactive approach to preventing grape diseases from destroying a crop or the vineyard itself.

Grape diseases manifest themselves in different ways. Fungi, viruses and bacteria & phytoplasmas can infect a grape plant. Each of the before mentioned classifications of microorganisms can create major headaches for the vintner. Thankfully there are solutions for most of the various grape diseases. For fungi the following diseases are common to grapes: powdery mildew, downy mildew, phomopsis cane and leaf spot, black rot, botrytis bunch rot, anthracnose, bitter and ripe rot, eutypa dieback. For viruses the following diseases are common to grapes: leafroll, tomato ringspot & tobacco ringspot. For bacterial & phytoplasmal microorganisms, the following diseases are common to grapes: crown gall, pierce's disease and grapevine yellows. The following paragraphs describe each disease as well as the remedies available to the vintner.

Fungal diseases present a broad spectrum of infections that can cause significant damage to the grape plant. Thankfully, most fungal diseases are perennial and as such plants do not typically stay infected between seasons. This leads to controlling fungal infections by eliminating spores that are present at the start of the season. The removal of diseased tissues where spores may be present, general sanitation procedures, proper canopy management for humidity control and the proper use of fungicides are all methods used to control fungal infections.

Powdery mildew is perhaps the most widespread fungal disease of grapes. Found worldwide, powdery mildew is caused by the fungus Uncinula with Vitis vinifera being extremely susceptible. Left unchecked, powdery mildew can destroy berries, retard vine growth, degrade the quality of berries, and weaken the grape plant overall. Powdery mildew manifests itself as a white or gray powder on the leaves of the grape vine. It can infect any green tissues of the grape vine with these tissues often turn brown or black in color. Removing spores from the vineyard, designing the vineyard with airflow in mind and the use of fungicides all work well at eliminating the threat of powdery mildew.

Downy mildew is a disease caused by the organism Plasmopara viticola. This particular infection manifests itself differently in different environmental conditions. Downy mildew can be almost non-existent in dry weather only to explode when humidity increases. The mildew causes defoliation leading to non-ripening fruit and huge crop losses. Lesions typically appear on leaves as yellow or red-brown circles. Improved air circulation and fungicide applications go a long way in controlling downy mildew.

Phomopsis cane and leaf spot is a disease that infects the fruit and stem bunches of the grape plant. If phomopsis cane and leaf spot is aggressive enough, it can lead to huge crop losses. Phomopsis cane and leaf spot is caused by the Phomopsis viticola fungus and is generally more damaging to native American grape varieties. Sanitation of the vineyard along with the pruning of infected canes goes a long way in controlling phomopsis cane and leaf spot. Fungicide application programs are also used in controlling phomopsis cane and leaf spot.

Black rot is a serious disease most prevalent in the vineyards of eastern North America. Caused by the Guignardia bidwellii fungus, black rot will infect all green tissues of the grape vine. Leaves become infected and display small brown and black lesions. Defoliation usually occurs with later disease cycles infecting the grapes. The end result of the grape infection includes shriveled up inedible berries that resemble hard raisins. Sanitation of the vineyard along with fungicide applications is critical in controlling this disease.

Botrytis bunch rot is a destructive infection that can lead to great losses for the vintner. The fungus Botrytis cinerea is responsible for negative botrytis bunch rot disease as well as the positive Noble Rot condition. With botrytis bunch rot, large brown lesions appear on leaves with the infection eventually spreading to the berries. Infected berries develop a gray mold appearance and may also shrivel up. Controlling the disease includes selecting grape clones with looser grape bunches. Proper airflow and fungicide applications are required as well.

Anthracnose is a harmful disease that affects a handful of grape varieties. Caused by the Elsinoe ampelina fungus, anthracnose produces lesions on shoots and leaves that are round and tan colored. Berries eventually become infected with lesions of ash gray centers surrounded by a black outline. The use of lime sulfur goes a long way in preventing spores from overwintering. Fungicide applications are also used to control anthracnose.

The bitter and ripe rot diseases infect mature fruit and are caused by two different fungi. Melanconium fuligineum is responsible for bitter rot while Colletotrichum gloeosporioides and Colletotrichum acutatum are responsible for ripe rot. Symptoms between bitter and ripe rot are similar with berries turning brown with concentric rings on small fruiting bodies. Berries eventually fall off the bunch or shrivel into dark hard inedible berries called mummies. Sanitation practices and fungicide spraying programs are used to control this disease.

Eutypa dieback is a fungal disease caused by the Eutypa lata fungus. Described as a chronic canker-type disease, eutypa dieback effects grapevines throughout the world. Cankers--areas of the trunk that are damaged--are a characteristic of this disease and can be somewhat inconspicuous. A vintner may have to remove some bark to discover an infected area. Left untreated these cankers will produce a toxin that damages the foliage of the grape. The end result of this disease is death to the affected trunks of the grape vine. As there are no fungicides to combat eutypa dieback, prevention of the disease is critical. The removal of cankers and other infected vines is essential. Infected grapevine tissues should then be burned to kill off any spores.

Several viral diseases are common to grapevines worldwide. The good news with viruses is that they are seldom spread to vineyards by natural means. Viruses are typically found in infected planting or propagating material. It should be noted that no sprays or treatments are available for virus control. There are some rootstocks that have shown resistance or tolerance to viruses, and thus new vineyards can be developed with these rootstocks. Once a grapevine is infected with a virus, the virus cannot be removed.

Two viruses that affect grape vines are leafroll and tomato/tobacco ringspot. With leafroll, tissues between the veins on leaves turn red with the margins of the leaves turning downwards. The disease does not kill the vine but causes significant damage. Grape yields can drop by 20% due to the leaves being damaged. The grapes themselves may also be sub par and ill suited for winemaking.

Tomato/tobacco ringspot infections are a seed transmitted disease affecting the vineyards of the northeast United States. Vitis vinifera grape vines are somewhat resistant to the Tomato/tobacco ringspot disease. Vitis labrusca vines are completely resistant. Infected varieties include hybrid varieties Baco noir, DeChaunac and Chelois. Symptoms of the tomato/tobacco ringspot include poor development of berries and small-distorted leaves.

Several bacterial and phytoplasmal diseases currently affect vineyards. The Crown gall disease is caused by the bacterium Agrobacterium vitis and is typically found on freeze damaged Vitis vinifera varieties. Gall like forms appear on the trunks of infected vines. These galls negatively affect the vascular system of the vine and if severe enough can deteriorate the health of the vine. Crown gall is controlled by matching grape varieties to specific site characteristics (such as varieties that are more susceptible to cold.)

Pierce's disease is a destructive disease found in warm weather regions of the United States. Symptoms of pierce's disease include delayed bud growth, retarded shoot growth, leaves dying, premature fruit coloring and the degradation of the vines root system. Infection is caused by the bacterium Xylella fastidiosa and is transmitted by insect. The use of insecticides to kill transmitting insects before infection is one method of controlling pierce's disease. The best way to control pierce's disease it to not plant vineyards of susceptible vines where the disease is established.

Grapevine yellows diseases occur worldwide with regional variations being recorded. Phytoplasmas are responsible for the grapevine yellows diseases and are transmitted by insect. In the first year of the disease, very few shoots of the vine appear to be infected. The disease progressively gets worse in the following years. Symptoms of grapevine yellows include withering of grape clusters, the rolling and yellowing of leaves, and the failure of shoot stems to develop properly. The disease can be managed by avoiding the use of susceptible varieties in high-risk areas.

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