LERGREC | Plant Pathology | 2005 Grape Disease Summary You are here

2005 Grape disease development at the Lake Erie Regional Grape Research and Extension Center, North East, Pennsylvania

The following is a summary of observations concerning the weather during the 2005 season and how it affected grape disease development at the Penn State, North East PA experiment station.

Weather and Phenology:
The 2005 growing season started out cold, dry, and late. Punctuated by infrequent deluges throughout the season, 2005 ended up as one of the warmest seasons on record. Cold spring conditions prolonged dormancy until the second week of May (almost 2 weeks behind our 8 year average). As we entered June, the cold weather relented and temperatures rose to above normal. The warm temperatures accelerated grapevine development, and bloom for most varieties began at the end of the second week to the third week of June, not far from average. Concord veraison occurred around August 23 (about average) and we harvested a larger than average Concord crop of very good quality.

From May 1 through September 30 we recorded a total of 28.72 inches of rain, above our 10 year average of 24.15 inches. So why were diseases relatively easy to control in 2005? At least two reasons: over 80% of that 28.72 inches fell during the second half of the season, when Concord fruit were resistant, or becoming so, to most diseases. In addition, over two thirds of that 28.72 inches fell during only six separate rain events, again, mostly during the second half of the season. Infrequent rain meant fewer infection periods and more opportunities to keep vines protected, and inoculum levels down, with fungicides (with longer lasting residues) particularly at the most critical times for fruit infection. The dry weather from May 1 to July 15 would have lasting effects on disease development in 2005. Rainfall for May 1 - July 15 was lower in 2005 than in any other year for the last ten years.

Powdery Mildew:
Weather conditions in May and June 2005 were not favorable for grape powdery mildew primary infection. Only three weak primary infection periods occurred after budburst during May. Cool temperatures for those infection periods were marginal for infection and two of the three infection periods occurred when availability of host tissue was minimal (2-3 days after budburst). Two more weak primary infection periods occurred in June before bloom. The total of only 5 primary infection periods from budburst to bloom, was far below our 6 year average of 11.5. The last of the June primary infection periods occurred as a cluster of 3 rain events from June 15-17 (about mid bloom). California-like conditions characterized the 2-3 week fruit susceptibility period following bloom. The lack of primary infection reduced our usual load of secondary inoculum (which doesn't require leaf wetness for spore release) to fuel an epidemic. As a consequence, fruit infection was very minimal and easy to control in 2005. A block of Concords at the North East research station that has consistently received only one pre-bloom and one post bloom fungicide application for powdery mildew had the lowest level of fruit disease in at least the last 6 years. The figure below illustrates how Concord fruit infection levels (North East, PA) have steadily declined over the last four years (under a minimal fungicide program) to extremely low levels. Leaf infection was also very late and very minimal in 2005.

Downy Mildew:
2005 was unfavorable for downy mildew development. The pathogen causing this disease (Plasmopara viticola) is profoundly dependent on rainfall and the presence of wet plant surfaces for initiation and completion of its parasitic life cycle. Rainfall providing the means for this pathogen to jump from the soil to the canopy, was in short supply during critical parts of June and July, and for many growers, this disease literally did not get off the ground this year. The 5-6 leaf stage, marking the start of downy mildew season, occurred about June 7 this year, (two weeks later than last year but about a week earlier than in 2003). First sporulation of the pathogen, which we commonly observe on suckers in our Chancellor vineyard, occurred on June 16 (compare with first observance on suckers on May 18 last year and June 12 in 2003). We recorded only 1.95 inches of rain in June and three quarters of that came during one rain period from June 15-17, when, according to DMCAST, the first infection period occurred. The next rainfall event (which was "weak") occurred on July 4-5, nearly 3 weeks later. Heavier rainfall occurred during the second half of July, sparking numerous infection periods, and August was marked by two major wetting periods from August 10-14 and 30-31 (Katrina). But, most vineyards had maintained tight control of the disease and perhaps there was little inoculum for downy mildew to build on during this period. Throughout the season, downy mildew was rare in our vineyards, and the absence of grower reports regarding this disease indicates that, overall, downy mildew did not present much of a problem for Lake Erie grape growers in 2005.

Black Rot:
The lack of moisture early in the season was also detrimental to black rot development. The first leaf infection symptoms (lesions) were observed on Concord vines in two vineyards on June 8 and only on the two to three oldest leaves of shoots. This placed the first infection period at sometime in mid-late May as leaves generally must become infected within a week or so after emerging in order to show symptoms. Symptoms can appear about14 days after infection. Wetness periods in June were very brief until June 16 (mid bloom) when temperatures dipped into the 50s and drying was slow. However, the low temperatures also reduced the activity of the pathogen and the length of the wetting period required for infection. No more opportunities for infection occurred until July 4-5 which was considered a weak infection period. The wetter second half of July provided conditions for several, more extensive, infection periods at the end of the fruit susceptibility period, and black rot cluster infection did appear in early August in a few Lake Erie region vineyards where problems with black rot occurred last year. In general, vineyards that had good coverage with effective materials through early July ended up in good shape.

Overall, low disease levels in 2005 mean low inoculum levels in spring of 2006. But disease pressure depends on several factors in addition to overwintering inoculum load, and no one can say that disease will be "easy" to control in 2006. As always, disease pressure will depend, to a large extent, on the weather.