PPDL Picture of the Week

August 6, 2018

Grape Anthracnose (Elsinöe ampelina) and Black Rot (Guignardia bidwellii)

Bruce Bordelon, Professor, Department of Horticulture and Landscape Architecture

As dry as it’s been in Indiana this year, it’s surprising that we have much of a fruit disease problem. However, remnants of tropical storm Alberto hit the state late in May bringing rain along with the well above average temperatures. Those were perfect conditions to cause an outbreak of diseases in grapes. The two diseases we are commonly seeing are anthracnose and black rot.

Grape anthracnose, aka black spot or Bird’s eye rot is not uncommon, but is seldom widespread except in very wet years. Anthracnose can infect all green grape tissues. Symptoms first appear early in the year on the first few internodes of new shoots. They are deep lesions with dark margins and a gray center. If the disease spreads to young tissue, it can distort and kill the shoot tips, giving the shoots a burned appearance (Picture 1).  Leaf lesions often cause the leaf to distort and curl. Centers of the spots often fall out, leaving a shot-hole appearance (Picture 2). The disease spreads to developing berries. Berry lesions appear as a dark spot with a gray or tan center, giving the disease its common name, Bird’s eye rot (Picture 3). Bird’s eye rot is mostly cosmetic, as it does not affect the eating or processing quality of the fruit. Severe infection, however, can reduce vine vigor and yield.

Grape black rot is the most common disease of grapes in the Midwest. It is the main disease that our spray programs are designed to control. Black rot can cause complete loss of the crop if left uncontrolled. Black rot can infect leaves, tendrils and shoots, but is most common and most damaging when it infects berries. Leaves are susceptible any time during the growing season. Symptoms are characteristic tan spots with dark, often purple margins (Picture 4). Pycnidia form along the other edges of leaf spots, which is a characteristic of black rot. (Picture 5.). Berries are susceptible immediately after fruit set until about 4 to 5 weeks later. Once they reach the bunch closure stage they become resistant to infection. Even though the disease is still active and can infect other tissues, berries are essentially immune to infection by mid-summer. Unfortunately, it takes two to three weeks for symptoms to develop on berries after infection, so by the time you see any signs of disease, it’s too late to do anything about it. And spraying to control the disease at that stage is useless since the remaining berries have likely developed resistance.  Symptoms of black rot on fruit are a rapidly developing tan rot of hard green berries. Infections usually start on the side of the berry and rapidly consume the entire fruit (Picture 6). Pycnidia develop on the surface of the infected fruit and produce the asexual spores that spread the disease to other susceptible tissue (Picture 7). Within a couple of weeks, the tan berries shrivel into hard black mummies. Eventually the overwintering sexual stage of the fungus develops on the mummies. (Picture 8). These perithecia will release spores next spring to start the cycle over.

Managing anthracnose can be relatively easy. The best control method is to apply a delayed-dormant application of liquid lime sulfur or Sulforix (calcium polysulfide). This fungicide effectively kills the developing spores (primary inoculum) at the beginning of the season and prevents the disease from becoming established. If this spray is missed and the disease becomes established, control is more difficult. Lime sulfur will burn tender foliage so it must be applied just as buds are swelling, but before the leaves are exposed. After bud break mancozeb, captan and the strobilurin fungicides such as Abound or Sovran can provide some control and keep the disease from spreading. But the single application of lime sulfur usually provides nearly complete control. That’s why it’s important for growers with a significant problem this season to plan ahead for an early season fungicide application next year.

Managing black rot is more complicated. We focus mainly on protecting developing young fruit from infection so applications made just prior to bloom and repeated weekly for 3 to 5 weeks are most effective. Protectant fungicides such as mancozeb provide excellent control, but can be washed off by rain. The demethylation inhibitor fungicides also provide excellent control and are systemic and rain fast. One of these DMI products, Rally (active ingredient myclobutanil) is very effective and available to commercial growers. Many of the fungicides available to homeowners contain the same active ingredient so are an excellent choice for small growers. The real key to controlling black rot is correctly timing the sprays to be in place prior to a rainfall-induced infection event. Growers can learn more in our publication Managing Pests in Home Fruit Plantings (ID-146-W) https://edustore.purdue.edu/item.asp?Item_Number=ID-146-W

 

​Click image to enlarge


Picture 1: Anthracnose lesions on a shoot tip.


Picture 2: Anthracnose lesions on leaves, showing the shot hole symptom.


Picture 3: Anthracnose lesion on berry showing the bird’s eye symptom.


Picture 4: Black rot lesions on a grape leaf.


Picture 5: Close up of black rot leaf lesion showing ring of pycnidia.


Picture 6: Early black rot symptoms on berries showing start of rot on the side of the berries and rapid progression.


Picture 7: Development of pycnidia on black rot infected berry.



Picture 8: Grape cluster with many black rot mummies. Note that some berries also show anthracnose bird’s eye symptoms.