2020 Growing Season Recap

Bryan Hed, Lake Erie Regional Grape Research and Extension Center, Penn State University

With this blog I would just like to briefly reflect on the 2020 growing season as well as present a few updates from research and industry that may affect how we tackle 2021. To be sure, this past growing and harvest season had its own challenges, dealing with COVID-19 restrictions and staying healthy. I hear a vaccine is in the works for the near future that may help us get through this soon, possibly before bud break next year. That said, I would like to take a look back at 2020’s weather and how it may have affected grape disease development this year. In Table 1 we report monthly precipitation at various locations throughout the Commonwealth. 

Table 1. Monthly precipitation (inches) recorded from June to October 2020 at several PA locations (from http://newa.cornell.edu/).

 MayJuneJulyAugustSeptemberOctober
North East (Northwest)4.543.444.252.411.776.82
State College (North Central)2.565.441.161.962.452.95
Scott Township (Northeast)2.602.533.684.732.703.24
Pittsburgh airpt (Southwest)2.192.093.105.570.923.29
Biglerville (South Central)2.162.472.483.392.293.53
Breinigsville (Southeast)1.784.085.659.394.173.13
Allentown (Southeast)2.682.274.909.804.173.13
Bainbridge (Southeast)3.165.552.146.042.232.86
Philadelphia airpt (Southeast)2.203.215.548.534.204.10

For most locations examined, the first thing that struck me of course was the drier weather throughout most of state, for most of the season. One of the biggest challenges many PA growers below I90 have had to deal with in recent years, has been excessive rainfall and the downy mildew and late season bunch rots that come with it. In some years, growers may have had to throw ‘everything but the kitchen sink’ at controlling downy mildew and were running through the inventory of FRAC groups to control it AND try to manage resistance at the same time. And, when you’re running a small acreage farm, you may not be able to afford to keep an entire inventory of FRAC groups in your fungicide lineup, even with the minimum amounts of each pesticide you can buy. When you can no longer use mancozeb products (past 66 days before harvest) and Ziram isn’t as effective, you have Captan and copper. But Captan and copper can cause problems with fermentation if residues are too high at harvest, so you have to use them with caution late in the season. Ridomil Gold is wonderful but its only sold with mancozeb or copper with a 66 or 42 day preharvest interval. And, there’s materials like Revus, Ranman, Zampro, and the phosphorous acids, which are great, but they too have their preharvest intervals and must be rotated to manage resistance, or we’ll lose them altogether. Note too, I did not mention the strobilurins like Abound and Pristine, and Reason (another FRAC 11 material) for reasons I elaborate on below.

In 2020, the May, June, and July rainfall totals don’t look like they were too troublesome and in some locations it was actually quite dry. Still, some southern locations had a very wet August and September as far as total rainfall is concerned, but the timing of the rainfall was about the best you could hope for. For example, the high August rainfall at Breinigsville and Allentown was mostly in early August at the beginning or just before ripening began. September rainfall was concentrated at the very end of the month, after some varieties had likely been harvested. So even though rainfall totals for August and September for many locations in the Southeast were well above average, most of that rain fell along the edges or just outside the ripening period, when fruit are most susceptible to late season bunch rots.  So, despite the other challenges in 2020, it would appear that the weather was a little more cooperative for a change.

Since downy mildew and late season fruit rot management will continue to be a major challenge for PA growers in years to come, we recommend that every serious grower obtain a copy of the NY and PA Pest Management Guidelines for Grapes. A new, updated 2021 version will be hot off the presses early next year. You can purchase one through Cornell University press.

…And now for a couple of research updates

  1. Evaluation of a new fungicide for powdery mildew control

There’s a new sterol biosynthesis inhibitor fungicide called Cevya. Cevya is in the same FRAC 3 group as Tebuzol, Tebustar, Rally, Mettle, Rhyme, Viticure, Procure, Trionic, the difenoconazole products (Revus Top, Aprovia Top, Quadris Top, Inspire Super), and many, many more. It was approved for use in PA last year and is now approved for use in New York (except Long Island). 

     If it’s a member of an old chemistry class, what’s so special about Cevya??? Cevya is good to excellent on powdery mildew, from recent results of several trials in PA and NY. This is in contrast to some of the older members of this group that may have slipped a bit with respect to powdery mildew control over the past several decades, due to resistance. However, 2020 was the first year of trial work for black rot control with this product: like other chemistries in this group, Cevya provided excellent control of black rot on Concord and Chambourcin fruit, providing complete control of this disease when applied either before infection (as a preventative) or up to 5 days after infection (as a post infection, curative application). We plan to do more trial work with Cevya in 2021 to further confirm its powdery mildew and black rot efficacy. Unfortunately, there’s a caveat: Cevya is not currently labeled for use on labrusca or labrusca hybrid grapes….ONLY VINIFERA (read the label). However, label expansion is anticipated, so stay tuned! In two years of our trials to evaluate phytotoxicity to native and hybrid grapes (recall the caution with difenoconazole use on Concord and some hybrids), Cevya appears to present no injury issues to Concord, Niagara, Chancellor, Chambourcin, Vignoles, and Vidal grapes, even when applied multiple times on a 14 day schedule, at double the label rate, with a penetrating surfactant. However, we will have to wait for potential label expansion before we consider using Cevya on those natives and hybrids. 

2. Mechanized leaf removal in the fruit zone

We also continue to examine mechanized defoliation of the fruit zone (at ‘just before bloom’ and at ‘fruit set’) for effects on late season bunch rot development. Our results over the last several years utilizing air pulse leaf removal systems, have shown consistently that mechanical fruit zone defoliation (at either timing), as part of an IPM program with effective fungicides, will help to reduce late season bunch rots over fungicides alone, on susceptible varieties like Pinot Noir/Gris, Riesling, Chardonnay, and Vignoles. Trellis systems like ‘vertical shoot position’ and ‘4-arm kniffen’, that produce more two-dimensional canopies, tend to maximize the beneficial effects of mechanized defoliation over that of a high wire no-tie system that produces a more three-dimensional canopy. In other words, if you want to get the best bang for your buck from mechanized leaf removal, it appears the more upright, 2-dimensional trellis systems are the way to go. 

In 2020, we initiated a trial with Riesling, to examine 4 clones (239, 198, 110, and 90) and two hybrids of Riesling (Geisenheim and NY81) all interplanted and replicated (5x) in the same vineyard. Plots were 3 panels long (12 vines) and all were cane pruned and thinned to 96 shoots per panel (per four vines; 4 shoots per foot of row). We applied one of three treatments to each panel in each plot of each variety/clone for Botrytis bunch rot control; i) fungicides only (check), ii) fungicides plus mechanized defoliation in the fruit zone (MD) at pre bloom, and iii) fungicides plus mechanized defoliation in the fruit zone (MD) at just after fruit set (post bloom); Results of the early defoliation, utilizing the air-pulse system, can be seen in Figures 1A and B. 

     Results: For all varieties combined, MD reduced rots by 46-50% over fungicides alone, harmonizing with results from other trials with other varieties in past years. However, timing was not important; it didn’t matter if you applied the MD before or after bloom, both timings were equally effective at reducing bunch rot. Also, there was no effect of MD on yield. This is important because pre bloom fruit zone leaf removal by hand, has been shown to reduce cluster weights and yield in some of our past trials. That was not the case in this experiment with the air-pulse system; with all varieties/clones combined, yields were identical, regardless of treatment. 

     When we look at all individual varieties and clones for damage from bunch rot, we found that NY81 (a Riesling hybrid with Cayuga) was the least rot prone and had the largest yields. The other hybrid, Geisenheim (a Riesling hybrid with Chancellor), was most rot prone, largely a result of powdery mildew damage which was a big factor in the bunch rot that developed later from it. The four Reisling clones fell somewhere in between these hybrids, with clone 90 being most rot prone (and with the smallest yields) and 239 being least rot prone. Among the clones, 198 had the largest yield. 

Figure 1A. Riesling clone 239, immediately before pre bloom mechanized defoliation.
Figure 1B. Riesling clone 239, immediately after pre bloom mechanized defoliation.

3. And finally, tips to mull over during the dormant season for 2021 disease management

  • Hill grafted vinifera to protect scion buds from severe winter cold and insure a supply of suckers for trunk renewal. I hope we never get a repeat of the 2014-2015 winter, at least not in my lifetime. But there is no guarantee of that. Many in PA (and other northeastern states) that did not hill before that winter, suffered tremendous losses. If we have another severe cold event this winter and grafts are not protected, you can develop crown gall at the graft and worse yet, the whole scion can die leaving you nothing with which to renew a dead trunk. Hilling will not prevent cold damage to the trunk and the crown gall that sweeps in after it, but it will save the scion just above the graft. A vine that you’ve toiled to cultivate for many years, and that has developed a deep, powerful root system, can become worthless if the scion is killed. Yes, there is the possibility of field grafting in case the scion dies, but that too can be laborious and costly, and requires a certain set of skills that may not be readily available. 
  • Continue to keep detailed records of where diseases were worst; those are the areas where disease are likely to show up first in 2021. If you had an easier time with downy mildew this year, remember that the pathogen can remain in your vineyard for several years in the form of tiny overwintering structures that ‘rest’ on the vineyard soil. As next spring rolls around, begin scouting for downy by mid to late May. The first downy mildew infections can occur during rainfall (at least 0.1 inches of rain and 50 °F) a few weeks prior to bloom, when vines have developed about 5-6 leaves per shoot. Keeping records on where downy tends to be most problematic in your vineyards, will help you to focus your early scouting efforts for more efficient and effective fungicide applications. 
  • Understand the pros and cons of the downy mildew fungicides. The old standards like mancozeb (Penncozeb, Manzate, Dithane, etc), Captan, and copper formulations are effective and are great for multiple, back to back applications because they pose little risk in terms of the development of resistance, but they are not as rain-fast as the newer materials like Revus, Ridomil, Phosphorous acids, and Zampro, and may need to be reapplied more often under heavy and frequent rainfall conditions. In the other hand, the newer fungicides should not be used more than two or three times per season, and even though the label may permit it, we recommend you don’t make back-to-back applications of the same chemistry, among these materials. That may be fine for one year. But as you know, we can get hit with high downy mildew pressure for several years in a row, and making back to back sprays of the same chemistry, for multiple years in a row, can burn that chemistry out faster. And speaking of resistance, downy mildew resistance to the strobilurins (FRAC 11, which also includes Reason fungicide) is common in the northeast and this class of fungicides should probably not be relied upon anymore for control of this disease in many parts of Pennsylvania where these products have been used intensively over the past 10-20 years. This is especially important when considering what to spray during the critical fruit protection period, from immediately before bloom to about 4 weeks after bloom. 
  • Cultural measures to apply this winter to reduce overwintering sources of inoculum. Cultural controls are generally not a substitute for seasonal fungicide sprays, but they can make your spray program more effective. To avoid reinventing the wheel, here are some tips from an earlier blog; “During dormant pruning, remove all clusters not harvested and as much diseased/dead/old wood from the trellis as is practical. Throw this material into the row middle and chop it up, or better yet remove it from the vineyard and burn it. This is especially effective against Phomopsis and black rot. Also, upright training systems (like vertical shoot position) reduce the probability that pathogen spores will be splashed upward from cordons and trunk, into the fruit zone during rain”. 

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