Our extension plant pathologist Jason Brock offers the following recommendations based on Dr. Tim Brenneman’s research regarding nut scab:
June through July is a critical time for pecan scab management, as nuts are most susceptible during sizing. Nut scab development early in the summer will be much more devastating than late season infections. We have been getting more calls about fungicide selection, so I wanted to reinforce our recommendations. I will address two situations, depending on disease potential.
With Desirable or any variety with similar scab susceptibility, a rotation of Elast/Tin with Miravis Top is the strongest option. You could use consecutive applications of either, but keep in mind Miravis Top use restrictions include a 14-day minimum application interval and a maximum of 4 applications per year. In most cases, half rates for Elast/Tin are suitable. Research trials have shown that increasing the rate of Elast to 37 fl. oz. while mixed with Tin can provide better scab control, but only when disease pressure is high. Unless we get into a rainy summer, the half rates are the better option.
Cultivars with relatively low scab susceptibility provide more options for fungicide selection. In addition to Miravis Top, Elast and Tin (either in a mix or used stand-alone), other fungicides are suitable for these cultivars. Another tank mix combination that has provided good nut scab control is a Group 3 (DMI) + Group 11 (strobilurin). This combination is available in a number of premixed products. A newer option that we have recommended is a 3-quart rate of a phosphite. While the phosphites’ strength is in foliar disease control, a high rate application is suitable for nut scab control of certain cultivars with low scab susceptibility.
Aside from fungicide selection, important factors in scab control will be timely applications and good coverage. Pay close to attention to rain forecast and try to stay ahead of scab development; however, do not sacrifice good coverage by traveling too fast or trying to spray when weather conditions are detrimental to good coverage. Always remember to rotate chemistries regularly to manage fungicide resistance.