During the last week I have visited numerous fields and obtained additional samples through the Plant Disease and Insect Clinic at North Carolina State University that had anthracnose fruit rot. Earlier in April, it appeared that the incidence of anthracnose would be low and I worked directly with affected farms to determine the best course of action using available management strategies and registered products. Integrated management strategies were outlined last week's North Carolina Pest News. However, with the sudden occurrence of numerous farms reporting problems, there was considerable concern of widespread incidence. I have isolated Colletotrichum acutatum from infected plants, an aggressive pathogen not controlled satisfactorily with current options available to strawberry producers. Please see last week's note in the North Carolina Pest News that also differentiates anthracnose from Alternaria fruit rot. I have encountered substantial crown rot due to the C. acutatum strain, an unusual event. This phase is difficult to differentiate from Phytophthora crown rot. Send samples to the Plant Disease and Insect Clinic for a sure diagnosis since it would be unproductive to use fungicides in situations where it is not warranted.
Due to the incidence of anthracnose in the state, North Carolina Department of Agriculture and Consumer Services (through the Pesticide Section's Registration, Licensing and Information Unit) issued a crisis exemption for the use of Quadris Flowable, based on the recommendation of the North Carolina State University strawberry plant pathology specialist. I thank Agriculture Commissioner Meg Scott Phipps for her support in this matter and for the capable assistance of Lee Davis of the NCDA&CS Pesticide Section. The registration package submitted by the Department of Plant Pathology was acted upon within 7 hours. I requested that a general press release not be prepared since our growers are currently in full production. Therefore, we request that Cooperative Extension Agents, NCDA&CS field personnel, and other farm advisors ensure all interested farm clientele become aware of this action.
Quadris is a protectant fungicide and should be applied prior to or in the early stages of disease development. Applications may be made by ground only. Use sufficient water volume for adequate coverage (a minimum of 20 gallons of water is recommended). Apply Quadris at 7 to 14 day intervals at 6.2 to 12.4 fluid ounces per acre. Applications can be made up to day of harvest. Use the higher rate when conditions for disease epidemics are severe. No more than 3 sequential applications of Quadris should be made before alternating with fungicides that have a different mode of action. Do not alternate or tank mix with fungicides to which resistance has developed in the pathogen population. (Quadris is extremely phytotoxic to certain apple varieties; extreme care must be used to prevent injury to apple trees.) Do not apply more than 4.6 pints of product per acre per crop (1.2 pounds of active ingredient per acre).
Based on research conducted at North Carolina State University, Quadris tank-mixed with Captan has provided the best control of the anthracnose fruit rot stage. A full report of research trials (Pest Control Technology For the Control of Anthracnose and Botrytis in Strawberry) has been posted on the Internet through the Southern Region Small Fruit Center. This site will be good to review to learn about Quadris efficacy against anthracnose.
Quadris does not cure infected plants nor is it completely effective at preventing fruit rot. In fungicide trials Quadris has been found to reduce anthracnose fruit rot by about 60 percent once an epidemic starts. It should function as a helpful tool for growers who adopt an IPM program. Therefore, growers need to implement complementary disease management strategies. If anthracnose fruit rot occurs in a hot spot, the infected plants should be pulled immediately, placed in a bag or container, and removed from the field. Growers should also consider removing plants from rows immediately adjacent to the infected area. In cases where growers desire reduced fungicide usage, Quadris could be applied to plants surrounding such hot spots (e.g., 3 to 4 beds beyond the infected area). We have had good success in containing the problem using this strategy. However, the anthracnose pathogen can infect plants without showing symptoms until warm and wet weather occurs resulting in widespread fruit rot. A reduced risk recommendation, which requires more fungicide use, is to use Quadris or Quadris tank-mixed with Captan, applied as a general field application. Remove infected fruit from the field when practical to limit the build-up of spores. Never move from an infected area to a non-infected area without washing and changing cloths, especially if plants are wet. Such movement will lead to spread of the disease. I would also suggest that growers with know anthracnose problems apply Quadris before predicted rain events if it has not been applied in the last 7 days.
Quadris should not be used indiscriminately. Most growers in North Carolina have no or low risk of anthracnose and Quadris is not needed in such cases. Quadris has a specific mode of action and improper use will make the fungicide ineffective for the control of anthracnose if the problem pathogens acquire resistance. Resistance management is an essential component of using Quadris.
For more information, contact your local North Carolina Cooperative Extension Service office or Dr. Frank Louws at 919-515-6689.
The fungus Didymella bryoniae causes a disease known as gummy stem blight in cucurbits. Gummy stem blight is one of the most important diseases of cucurbits in the southeastern United States. The fungicide azoxystrobin (Quadris) was registered on cucurbits in 1999. Since then, it has been widely used to control gummy stem blight in North Carolina. Within the past two months, Syngenta personnel have collected several isolates of D. bryoniae that were not sensitive to azoxystrobin in the laboratory. These isolates were collected from areas of Delaware, Maryland and Georgia with intense cucurbit production where azoxystrobin did not control the disease at satisfactory levels. The company has recommended that azoxystrobin no longer be used in these counties.
Insensitivity to azoxystrobin is an important problem for cucurbit growers to be aware. According to the U. S. Environmental Protection Agency, strobilurins (e.g., Quadris, Flint, Sovran, Cabrio; the latter two are not yet registered) are reduced-risk fungicides (i.e., safe for handlers, consumers, and the environment). Consequently, they are being promoted as replacements for older fungicides. Because all strobilurin fungicides affect fungi in the same way, insensitivity to one strobilurin very likely means insensitivity to all strobilurins (cross resistance). This means that strobilurin fungicides, the only new, broad-spectrum fungicide class registered on cucurbits in the past two years, may already be of limited use against gummy stem blight.
What can growers in North Carolina and other unaffected areas do? Growers must follow a strict 1:1 rotation pattern (i.e., one application of Quadris (or Flint), followed by a different fungicide before applying Quadris (or Flint) again. The choice of a rotation partner depends on the target disease. For gummy stem blight, use chlorothalonil, mancozeb, or maneb. See the North Carolina Agricultural Chemicals Manual for rotation partners to control other diseases. Also, no more than four applications of Quadris or Flint may be made per crop each season. As a further precaution, Quadris should not be applied after there is more than 20 percent of the vine cover affected with gummy stem blight, which normally happens late in the season.
(Modified from: Keinath, A. Timely Talk about Insensitivity to Quadris. May 4, 2001.)
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Last modified on May 7, 2001 by Stephen J. Toth, Jr.