Management of botrytis gray mold in fall raspberries
Fall raspberries are more susceptible to gray mold due to normal seasonal weather conditions. Know what symptoms to look for and which fungicides can control gray mold.
Gray mold, caused by the fungus Botrytis cinerea, is one of the most important diseases affecting fall raspberries. Fall raspberries are usually at greater risk of infection than summer raspberries because of the prevailing weather conditions, such as lower temperatures, heavy dews and frequent precipitation. Cool, wet weather and heavy rains in the late summer and fall that keep the plants wet for extended periods are conducive to development of the fungus and infection of the fruit.
Typical symptoms include a brown discoloration of the fruit and the presence of a gray fuzzy mold, which can rapidly develop and spread to neighboring healthy berries. Symptoms tend to be more severe inside the canopy and on clusters that are closer to the ground. Even if berries look perfectly healthy at harvest, they can change to a moldy mass within 24 to 48 hours. To know how much disease pressure you have and assess the efficacy of your spray program, pick 10 or 20 random ripe berries and place them in a covered dish on moist paper towel at room temperature. If berries stay 90 percent free of visible mold for three days, they are in good shape.
Botrytis cinerea is a ubiquitous fungus that is able to grow and sporulate profusely on dead organic matter. It overwinters in old infected canes and plant debris. The spores are airborne and can travel long distances on the wind. When the spores land on plant surfaces, they germinate and can invade the plant tissues directly or through wounds. Overripe berries and bruised berries are particularly susceptible to infection. Latent flower infections, even though they do occur, are not as important in raspberries as they are in strawberries.
Cultural methods are very important for control of Botrytis gray mold. Choosing a site with good air flow can reduce humidity in the canopy considerably. Low-density plantings, narrow rows and trellising can also reduce a buildup of humidity. Good weed control and moderate fertilizer use to avoid lush growth are also important. Selecting a resistant cultivar or, at the minimum, avoiding highly susceptible cultivars will help to reduce the need for control measures. During picking, avoid handling infected berries since spores can be transferred on hands to healthy berries. Timely harvesting and rapid post-harvest cooling can also help to reduce losses to Botrytis gray mold.
Several fungicides are labeled for control of Botrytis in raspberries. Sprays close to harvest help to reduce post-harvest rots. Switch (cyprodinil + fludioxonil) is a reduced-risk fungicide with excellent systemic and protectant activity against gray mold. It has a 0-day pre-harvest interval (PHI). Another good option is Elevate (fenhexamid), which is a reduced-risk, locally systemic fungicide with a 0-day PHI. Since these fungicides are in different chemical classes, they can be alternated for fungicide resistance management. My recommendation is to save Switch and Elevate for critical sprays, for example, during wet periods and for sprays closer to harvest. Other fungicides that may be used in the spray program are Pristine (pyraclostrobin + boscalid: 0-day PHI), Captevate (captan + fenhexamid: three-day PHI), Captan (captan: three-day PHI), Rovral (iprodione: 0-day PHI) and Nova (myclobutanil: 0-day PHI). To improve the efficacy of Rovral, an adjuvant should be added. Pristine and Nova also provide excellent control of late leaf rust, which sometimes infects the leaves and fruit of fall raspberries.
Dr. Schilder's work is funded in part by MSU's AgBioResearch.