Phomopsis Cane and Leaf Spot

For many years, the Eastern grape industry recognized a disease called "Dead-arm" which was thought to be caused by the fungus Phomopsis viticola. In 1976, researchers demonstrated that the dead-arm disease was actually two different diseases that often occur simultaneously. Phomopsis cane and leaf spot (caused by the fungus Phomopsis viticola) is the new name for the cane and leaf-spotting phase of what was once known as "Dead-arm" Eutypa dieback (caused by the fungus Eutypa lata) is the new name for the canker-and shoot-dieback phase of what was also once known as "Dead-arm" The name "Dead-arm" has been dropped. Growers should remember that Phomopsis cane and leaf spot and Eutypa dieback are distinctly different diseases and their control recommendations vary greatly.

Disease incidence of Phomopsis cane and leaf spot appears to be increasing in many vineyards throughout the Midwest. Crop losses up to 30% have been reported in some Ohio vineyards in growing seasons with weather conducive to disease development. The most commonly observed symptoms are on shoots where infections give rise to black elliptical lesions that are most numerous on the first 3-4 basal internodes. Although this phase of the disease can appear quite severe, crop loss due to shoot infections has not been demonstrated. Heavily infected shoots are more prone to wind damage.

Although shoot infections may not result in direct crop loss, it is important to remember that lesions on shoots serve as an extremely important source of inoculum for cluster stem (rachis) and fruit infections in the spring. Rachis and fruit infection is the phase of the disease that causes most crop loss.

Phomopsis Cane and Leaf Spot Disease Cycle (Figure 67)

The fungus overwinters in lesions or spots on 1- to 3- year-old wood infected during previous seasons. It requires cool weather and rainfall for spore (conidia) release and infection. Conidia are released from pycnidia (fungal fruiting bodies) in early spring and are spread by rain to developing shoots and leaves. Shoot and leaf infection (Figure 68 and 69) is most likely during the period from bud break until shoots are 6 to 8 inches in length. Lesions appear 3 to 4 weeks after infection. The critical period for fruit and rachis infection (Figure 70) is also early in the season. The rachis and young fruits are susceptible to infection throughout the growing season; however, most infections appear to occur early in the growing season. The fungus does not appear to be active during warm summer months, and most or all of its primary inoculum is probably released and expended early in the growing season. Thus, the critical period to provide fungicide protection for fruit infection is probably from when the clusters are first exposed until 2 to 3 weeks after bloom.

Figure 67: Phomopsi cane and leaf spot disease cycyle. We wish to thank the New York State Agricultural Experiment Station for use of this figure.   Figure taken from Grape IPM Disease Identification Sheet No. 6.

Figure 68: Phomopsis cane and leaf spot on internodes and rachis.

Figure 69: Phomopsis symptoms on grape leaf.

Figure 70: Phomopsis fruit rot on grape.

The tiny green fruits that are infected during this critical period may appear to be normal. The fungus remains inactive in these fruits as a "Latent" infection. Not until the fruit starts to ripen near harvest does the fungus become active and cause the fruit to rot. Therefore, fruit rot that appears at harvest is probably due to infections that occurred during or shortly after bloom.

Berry infections first appear close to harvest as a light-brown color. Black, spore-producing structures of the fungus (pycnidia) then break through the berry skin and the berry soon shrivels.

At this advanced stage, Phomopsis fruit rot can be easily mistaken for black rot. Growers should remember that the black rot fungus does not infect berries late in the growing season. Berries become resistant to black rot infection by 3 to 4 weeks after bloom. Fruit infection by Phomopsis generally does not appear until this time. Although the fungus does not appear to be active during the warm summer months, it can become active during cool, wet weather later in the growing season.

Organic fungicides (sulfur and copper) applied during the growing season are not highly effective for control. A dormant application of liquid lime sulfur may be beneficial in control.