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Section 29: Controlling Muscadine Grape Diseases

Muscadine grapes are native to and grow in the southeastern United States, a warm humid region with considerable potential for plant disease. Most of the diseases of muscadine grapes that require control are caused by fungi and can be controlled with various degrees of success with timely fungicide applications. Current fungicide recommendations for control of muscadine grape diseases are available from local Extension offices.

There are several vineyard practices that can significantly improve the results of any fungicide program, particularly those aimed at control of fruit rot diseases. Two of these practices involve the design of the vineyard itself and must be considered well before planting. One is the trellis system. In middle and north Georgia the Geneva Double Curtain (two wire “T” bar) trellis has the potential to increase yields about 30 percent compared to a standard single wire trellis. In practice, many muscadine growers have not liked this trellis system because of higher initial cost and higher maintenance cost than the single wire trellis. The two wire trellis also presents problems in disease control. Where a two wire Geneva Double Curtain is used, there is a much denser foliar canopy between the outside of the vine and the inner portion of the vine where a large percentage of the fruit develop. This foliage provides a barrier that tends to trap and hold moisture and is difficult to penetrate with fungicides. Both factors make fruit rot diseases of muscadine grapes more difficult to control where a Geneva Double Curtain trellis is used. Because of this increased difficulty in disease control, a Geneva Double Curtain trellis is not recommended in any area where the combined average rainfall for July and August exceeds 11 inches.

The second factor to consider is choice of irrigation system. An overhead irrigation system also wets fruit and foliage each time it runs. This increased wetness benefits pathogens of fruit and foliage in the same manner as rainfall. That is, it splashes the fungus spores around and provides moisture needed for infection. An irrigation system operating below the vines could provide water to the plants just as well without enhancing conditions for disease development. In areas where frost protection is needed, an overhead system can be installed in conjunction with an under vine summer irrigation system.

Another practice which should aid in disease control is to strip fruit from vines at the end of the season. We have consistently observed that growers who supplement their fresh fruit business with sale of grapes at the end of the season for juice products have less fruit rot problems than growers whose market is fresh fruit only. In harvesting for processing, mechanical harvesters strip all remaining fruit from the vines. In most fresh market only operations, tons of rotting fruit are left in the field on pollinizer cultivars and on vines incompletely harvested as the demand for fresh fruit weakens. It has also been observed that some fruit rot diseases such as ripe rot overwinter better in rotted fruit left in vines than in rotted fruit on the ground. Putting fruit on the ground provides distance between spores produced on old fruit and the current fruit crop. It also exposes old fruit to attack by an array of insects and secondary fungi, etc. not present on the vines. Removing rotted fruit in mid-winter during pruning would be a suitable alternative to end of season vine stripping.

Good weed control can also relate to improved disease control. Allowing weeds to grow up under or around the vines will tend to trap moisture and restrict air movement under the vines. This condition is very favorable to disease development.

Disease control in muscadine grapes generally require use of fungicides most years. Control of black rot during bloom, powdery mildew, and angular leafspot is not difficult. Control of bitter rot, ripe rot and Macrophoma rot is more difficult. Ripe rot sprays need to begin shortly after bloom when small grapes are formed. Macrophoma rot and bitter rot sprays need to begin as soon as grapes first show signs of ripening. Waiting to see disease can reduce control of Macrophoma rot about 50 percent. The effect of these fungicide sprays can be enhanced by use of single wire trellises, below vine summer irrigation systems, vineyard sanitation and weed control.

Insects likely to damage grapevines or grapes include grape berry moth, grape root borer, and aphids. Your county Extension office can recommend proper control measures for these pests.

Section 30: Harvesting and Marketing