The start of the 2015 growing has been wetter than normal for many pecan growers in portions of Oklahoma, Central and Eastern Texas, the Central Gulf Coast and middle Georgia (Fig 1.). The National Weather Service has declared that El Nino is present, with a 70 percent chance of continuing through this summer and 60 percent chance to endure through autumn.
El Nino tends to increase precipitation in the Southeast during the cool season (winter/spring), with lesser effects on summer rainfall. The 3-month outlook for precipitation issued by NWS on April 16, predicts above-average rainfall in South Georgia and Florida, with more normal rainfall elsewhere in the Southeast (Fig. 2). Rainy periods in the early growing season intensify problems with pecan scab, the most important fungal disease of pecan leaves, stems and nuts (see photo). Rainfall complicates scab control from 3 angles: 1) increased wetting of foliage and nuts means better conditions for fungal infection and epidemics, 2) fewer favorable opportunities to spray fungicides and more difficulty getting spray equipment into the orchard, and 3) more grass and weed growth, which prolongs drying time and raises humidity in the orchard. For these reasons, pecan growers in the Southeast are now on the defensive against scab.
The effects of scab include reduced leaf growth, leaf drop, stem dieback, nut drop, reduced nut size and reduced kernel percentage. The damaging period is from budbreak to shell hardening. Leaves are at risk when they are soft and expanding. Glossy, toughened leaves are not susceptible. Shuck tissue can be attacked throughout the growing season, but is of greatest concern during the rapid nut sizing period in June and July. Shuck infections after shell hardening are usually not economically damaging.
Pecan scab is a “polycyclic” fungus, meaning it has numerous infection cycles each season. A wet April means more inoculum and thus more potential for infections and crop damage in subsequent summer months. Brenneman (2005) demonstrated that early fungicide sprays during the pre-pollination period are critical to managing scab later in the summer in a year of high pressure, and less so if the pre-pollination period is dry. A study by Sparks, et al. (2009), in Georgia, found that rainfall in April when tabulated with rainfall from June to August gave the best prediction of scab effects on nut size. Above-average rainfall in April and May sets up pecan orchards for a pitched battle with scab if rainfall in June, July and August continues to be average or above average.
Scab will occur on susceptible pecan varieties, when the fungus is present and the conditions are favorable. There is no doubt about the presence of the fungus in all pecan orchards in the Southeast. Pecan cultivars vary widely in susceptibility, and thus control efforts must be greater for more susceptible cultivars like ‘Desirable’. Scab infection should be thought of as a 2-step process:
Step 1 – Germination of scab spores requiring free moisture on the plant tissue surface (Latham & Rushing, 1988).
Step 2 – Growth of the fungus into the pecan tissue, which requires dissipation of that free moisture but high relative humidity (Yates, et al., 1996).
Rain, fog or dew during warm weather that wets the leaves and nuts for 10-36 hours, followed by gradual drying and high relative humidity are ideal conditions for disease development. When such conditions occur, leaves and nuts of susceptible varieties will be infected unless a fungicide application intervenes.
So what can growers do in the months of June, July and August to make their best defense against scab, especially after a rainy pre-pollination period?
The cornerstone for nut scab control continues to be TPTH (Super Tin, Agri Tin). Dodine (ELast 400) is also effective in the summer, either tank-mixed with TPTH or used in alteration with it. Brenneman (2014) described improved performance of reduced rate tank mixes with TPTH and Dodine over full rate alternation. Phosphorus acid products (Phostrol, Viathon) have demonstrated some potential for improving scab control when used as a tank mix partner. Fungicides should not be over-used, which may lead to resistance, but in high pressure years, it is also important to not skimp on application rates. The Univ. of Georgia Commercial Pecan Spray Guide is an invaluable source for fungicide selection and identification of appropriate application rate.
Although the first tropical storm of the season—Ana—made landfall in the U.S. 3 weeks before the hurricane season officially started, El Nino conditions tend to reduce hurricane activity in the tropical Atlantic Ocean, as was seen in 2009. So while April rains may have increased the effort and expense of controlling pecans early in 2015, let’s hope the late part of the season is peaceful with regard to both wind and rain.