NDSU Creates Fungicide Resistance Strategy

FARGO - Jun 1/07 - SNS -- Aschochyta blight is becoming resistant to the fungicide used to control the disease, with the result growers need to change their crop management strategies, say North Dakota State University (NDSU) researchers.

In a new fact sheet, Kiersten Wise, Neil C. Gudmestad and Sam Markell, North Dakota State University (NDSU) Department of Plant Pathology, and Kent McKay North Central Research Extension Center, Minot write as follows:

Managing the disease Ascochyta blight on large kabuli chickpea often is a challenge for producers. Ascochyta blight is caused by a fungal pathogen and can result in yield and quality losses if conditions are favorable for disease development. A fungicide program of three to four fungicide applications usually is needed through the growing season.

The strobilurin fungicides (QoI), such as Headline and Quadris, have been very effective in reducing the disease in past years.

However, recent lab tests indicate that spores from some 2005 and 2006 Ascochyta blight samples collected in North Dakota can germinate in the presence of high concentrations of Headline and Quadris.

Preliminary experiments also indicate that applications of QoI fungicides may not reduce chickpea disease caused by fungicide-resistant spores. These data support observations by some chickpea producers that QoI fungicides are not as effective as they used to be in controlling Ascochyta blight and indicate the pathogen causing Ascochyta blight is resistant to QoI fungicides.

These results have led researchers at NDSU to recommend that QoI fungicides not be used on chickpea in North Dakota in 2007.

To reduce QoI fungicide resistance and still manage disease, following these guidelines when managing disease in this year's crop is critical:

Use certified, disease-free seed of an Ascochyta blight-resistant cultivar. Even low amounts of seed infection (less than 0.3 percent) can cause seedling infection and be a source of disease spread throughout a field. Chickpea seed can be treated with thiabendazole (LSP or Mertect) for management of seed-borne Ascochyta. This seed treatment is separate from the metalaxyl or mefenoxam (Apron or Allegiance) seed treatment used for other soil-borne diseases, such as Pythium.

Apply a preventative fungicide prior to flowering and disease development. Chlorothalonil and maneb (Manex) can be used to protect chickpea foliage prior to Ascochyta infection. Rates and instructions for use of Manex can be found on a 2ee specimen label for use on Ascochyta blight on chickpea.

Scout fields regularly to determine the onset of Ascochyta blight. Applications of prothioconazole (Proline; Group 3) or boscalid (Endura; Group 7) can be applied at flowering or the onset of the disease. Endura and Proline have different modes of action from each other and also from the QoI fungicides and have managed Ascochyta blight effectively in field trials. A fungicide spray program that rotates these fungicides will reduce the selection pressure on Ascochyta blight that leads to fungicide resistance.

Consider tank mixing Endura with a protectant fungicide, such as chlorothalonil or Manex, for added protection to foliage not infected with disease. Currently, Proline is not labeled for tank mixing, but studies are being conducted to determine if this fungicide can be used in mixes in the future.

Apply fungicides at label rates and follow label restrictions.

Small kabuli and desi-type chickpea generally are less susceptible to Ascochyta blight and usually only one or two fungicide applications are needed to manage the disease.

To reduce QoI fungicide resistance and still manage disease, follow the previous guidelines when managing disease in this year's crop. In a two-spray program, preventative fungicides, such as chlorothalonil and maneb (Manex), should be applied prior to flowering and disease development, followed by Proline or Endura at flowering or pod-set.

If conditions are dry and low levels of disease are present, only one fungicide application likely will be needed during the growing season.

To stop or slow the progress of fungicide resistance, and in the hopes that the QoI fungicides can be used again in the future for Ascochyta disease control, avoiding the use of any QoI fungicide on chickpea alone or in combination with other chemistries this year is important.