potato 5-9-12 CLEAN - Vegetableipmasia.org

ORGANIC POTATO PRODUCTION sheetUH (Reference 61), HUdisease cycleUH (Reference 62), HUcontrol optionsUH (Reference 63), HUorganic management optionsUH (Reference 63 B).Relative Risk: This disease occurs sporadically, but is totally devastating when present. In very wet cool weather, infectionscan spread quickly, leading to 50% or greater reductions in yield even with copper sprays, and complete yield loss if nocontrol measures are taken. Hot weather slows disease progress.Management Option Scouting/thresholds Site selection Crop rotation Resistant varieties Seed selection/treatment Recommendation for Late Blight Scout fields weekly for late blight symptoms. A forecasting system for late blight is available for some New York locations through the Network for Environment and Weather Awareness Potato late blight forecaster (Reference 4). The forecasting system has two parts: early season prediction of first appearance of late blight, and subsequent recommended spray intervals if late blight is present. Both parts use a measure called severity values (SV), which is calculated using hours of relative humidity over 90% and average temperature during those hours. Late blight is first expected to appear 1-­‐2 weeks after 18 SV have accumulated, starting with the emergence of green tissue from the source of late blight inoculum. The source of inoculum could be plants growing from infected tubers in a cull pile, volunteers growing from infected tubers that survived the winter, or infected seed tubers. Start scouting soon after 18 SV have accumulated if a late blight forecast is available for your area, or when potatoes are 4-­‐6" high. Conventional farmers begin applying fungicides at this point and maintain coverage until harvest, adjusting spray intervals to reflect weather conditions as described below. If late blight is found early in the season it may not be possible to control it adequately using approved copper products, and the field may need to be disked under. Track where late blight has been found in NY and monitor potential sources of late blight spores from off your farm at Cornell’s Weekly Late Blight Update (Reference 64). If late blight is found in your county or adjacent counties and you choose to use copper, apply an approved copper fungicide immediately even if late blight has not been found in your field. Coverage should be excellent throughout the canopy. Once fungicide applications have started, weekly accumulations of SV can help determine spray intervals. Cornell plant pathologists consider weather that accumulates six or more severity values in a week very favorable, indicating the need for a 5 day spray interval on potatoes; 3 -­‐ 5 severity values indicates moderately favorable weather and the need for a 7 day interval; less than 2 indicates unfavorable weather, when a 10 day interval may be used. Be aware that copper can build up in the soil. See Organic Resource Guide (Reference 2) and organic management options (Reference 63B) for more information about using copper fungicides. If present, harvest the crop early before it becomes contaminated. Harvest new potatoes and sell early, if possible. Avoid fields that cannot be effectively sprayed. Fields surrounded by trees that shade and slow air movement, or those remaining damp late into the morning are at higher risk for infection. This is not an effective management option. Potato varieties differ slightly in their susceptibility, but commercial varieties do not have useful levels of resistance. Late variety Elba has foliar resistance but not tuber resistance. Choose early maturing varieties that will allow early harvest. Infected seed potatoes serve as an important source of inoculum. Plant phytosanitary certified seed (See Section 7.1: Seed sources). Know your seed grower. Even state phytosanitary certified seed may have a low percentage of late blight. Obtain plant health certification from state certifying agency indicating if late blight was present in the field. Potatoes grown for seed must have no more than 1% late blight tuber rot. 332012