By: Bryan Hed
In many parts of the Eastern United States, 2015 will be remembered as one of the wettest fruit development periods in recent memory. Here at our Penn State research farm in North East PA, rain fell consistently, to the tune of about 2 inches per week, from late May through the middle of July! This level of wetness creates ideal conditions for diseases like downy mildew and black rot, of which there is abundant proof in our unsprayed check plots this year. Even growers of Concord grape, which has limited susceptibility to downy mildew, have been faced with downy mildew pressure they haven’t seen in a long time. Acreage under organic management where synthetic pesticides are prohibited, has, in some cases, suffered heavy losses from black rot. The wet weather also created perfect conditions for the establishment of latent infections of Botrytis during bloom and the early berry development period. These infections lay dormant in clusters until the ripening period (now), when factors related to high humidity, cluster compactness, and berry skin integrity, bring about the activation of these infections and the initiation of bunch rots.
More recently however, we have seen drier conditions prevail (only 2.62” rainfall over the past 7 weeks at our site) that have brought some relief, mainly from downy mildew (the danger of black rot fruit infections was past somewhere around the middle of July). The drier, sunnier conditions are more hostile to the survival of the downy mildew spores and can inactivate much of the sporulation so that an occasional wetting period will probably not amount to much additional leaf infection, at least not in vineyards that have kept this disease under control. Nevertheless, according to DMCast, (the downy mildew infection model developed at Cornell and loaded into the free NEWA website at http://newa.cornell.edu/), occasional wetting events may generate a downy mildew infection period in some locations IF there is active sporulation. This is why it’s so important to continue scouting leaves for the distinctive white ‘downy’ sporulation of this disease (Figure 1). Growers of susceptible varieties need to keep closely monitoring their vineyards for active sporulation and use that information in combination with the DMCast model on NEWA. If conditions turn wet more consistently, this disease can quickly spiral out of control, strip vines of their leaves and effectively end the season (and the ripening of canes for next year’s crop). So, despite the drier, hotter weather, the solid establishment of this disease across our region in June and July will likely mean that this disease will remain a serious potential threat for the rest of the season! If you find yourself trying to control this disease well into the ripening period, be aware that your list of chemical control options will start to become shorter as we get within 30, then 21, then 14 days of harvest, until in the end you’ll be left with some formulations of captan, copper, and phosphorous acid products.
Figure 1. Late summer downy mildew lesions on a mature leaf of Vitis labrusca ‘Niagara’. Note the absence of the more typical ‘oil spot’ symptoms that are observed on immature leaves in spring. Rather, lesions on mature leaves in mid-late summer take on a blockier appearance but still have the typical white downy sporulation underneath.
The threat of powdery mildew fruit infection (Figure 2) was over weeks ago. But as with downy mildew, leaves are susceptible to powdery mildew all season. However, powdery mildew leaf infection has been building rather slowly from our perspective along Lake Erie, and I see relatively little development of this disease on mature, exporting leaves of Concord and Niagara at our location. In fact the south side of our unsprayed east-west oriented rows are practically mildew free (sunlight is lethal to powdery mildew). Shoot tips, of course, are a different matter; we are seeing the classic distortion of new growth caused by heavy and rapid colonization by powdery mildew. This is nothing unusual, especially for this time of year. After more than 3 months of inoculum buildup in the air, unprotected new growth, which is highly susceptible to infection, is literally thrust into a hornet’s nest of powdery mildew spores and becomes infected as soon as it emerges. In vineyards that have largely controlled this disease to this point, infection of new growth is less severe and less rapid. These infections are also of much less concern (probably of no concern) in juice grape vineyards than in susceptible wine grape vineyards and will have little or no impact on the ripening of juice grape crops in the Lake Erie belt. And, according to work performed by Wayne Wilcox’ program, leaf infections that occur after Labor Day will probably not add to the burden of over-wintering inoculum for primary cycles next year; they likely don’t have time to mature before leaf fall. However, protection of new shoot and leaf tissue may still be important in wine grapes, especially Vitis vinifera. New growth is not only incredibly vulnerable to infection at this time (for the reasons stated above), but collectively serves as the perfect substrate for even more rapid generation of inoculum levels in the air; infected shoot tips represent an important source of late summer inoculum for powdery mildew. Sulfur is often the material of choice for late season control of powdery mildew; it’s relatively inexpensive, it’s effective, and you don’t have to be too concerned about the development of resistance. But too much sulfur on grapes during fermentation can lead to production of hydrogen sulfide which produces off aromas in the wine. When should sulfur applications be terminated before wine grape harvest? Of course, this depends to some extent on rainfall during ripening and sulfur rates. However, recent findings at Cornell (Kwasniewski et al. 2014) have shown that growers of red wines (for fermentation on the skins) should allow at least 5 weeks between that last application of sulfur and harvest (right about end of August for us in the northeast). With white wines (not fermented on the skins) late sulfur sprays are not thought, generally, to lead to issues with hydrogen sulfide. Other materials that have been used to successfully control this disease on leaves during ripening are things like monopotassium phosphate and formulations of potassium bicarbonates. These materials are not effective on heavy leaf infections but, according to one of my colleagues in Ontario, can work reasonably well if applied often (weekly?) to maintain relatively clean canopies, especially if you’ve exhausted your options of single site synthetic materials (Vivando, Quintec, Torino, Luna, strobies, sterol inhibitors).