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2004 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 2004 season and how it affected grape disease development at the Penn State North East experiment station.

Weather and Phenology:
Varying degrees of winter die-back occurred for a second year on some cold sensitive Vitis vinifera and French hybrid varieties at some locations in Pennsylvania and New York. However, 2004 was a year in which many cold sensitive varieties in Erie County, Pennsylvania, showed evidence of good recovery. Most varieties began breaking buds during the first few days of May and flowered during the second and third weeks of June, which placed vine development ahead of average. 2004 can best be described as WET. Our weather station recorded a total of 54.16 inches of rain for the 6 months of the growing season from May through October. That's an average of over 9 inches of rain per month (over 2 inches per week). The highest per-month rainfall was in July at 13.7 inches. It's no surprise that 2004 will be remembered as a year for black rot and downy mildew. Vine growth and development were generally ahead of schedule for May and June, and bloom occurred somewhat earlier than average. However, summer heat accumulation lost momentum in July and vine development proceeded at a more average to below average pace accompanied by large accumulations of rainfall. As the ripening period began, rainfall, although still heavy, became far less frequent throughout a 6-week period from early September to about the middle of October. This was also accompanied by lots of sunshine, greatly improving the harvest for the 2004 season.

Powdery Mildew:
Weather conditions in May 2004 were favorable for grape powdery mildew primary infection periods. Combined with the high levels of over-wintering inoculum, growers were expecting to have to deal with fairly high disease pressure. According to the basic primary infection model (>50 °F and > 0.1 inches of precipitation) there were up to 14 primary infection periods during May. Weather conditions dried up for the first half of June and growers found it easier to apply pre-bloom protective sprays. The early arrival of bloom in 2004 (in sharp contrast to 2003) probably helped move grape clusters through the susceptibility period somewhat ahead of rising disease pressure and powdery mildew fruit infection for most Concord growers remained at more typical levels. Disease pressure still remained relatively high for the more susceptible wine varieties with longer fruit susceptibility periods. The high frequency and volume of rainfall after bloom made it difficult for wine grape growers to maintain adequate fungicide residues on their vines, and the unseasonably cool temperatures during July may have reduced the rate at which fruit developed resistance to the pathogen.

Downy Mildew:
2004 was very favorable for downy mildew development, and crop loss was nearly total on unsprayed Chancellor vines at the experiment station. In fact, downy mildew remained a constant threat throughout the season, and susceptible varieties that did not receive protection throughout August and into September were often defoliated by the disease. Sporulation of the pathogen was observed on sucker growth in our Chancellor block on May 18, over 3 weeks earlier than in 2003. This sporulation underscores the importance of controlling early sucker growth in susceptible vineyards. However, the first symptoms of downy mildew on Chancellor clusters (epinasty) at trellis height were not observed until 3 weeks later on June 8. Shortly after this time, symptoms and signs of downy mildew were being observed throughout the vineyard. The summer months, which often bring relief from downy mildew through drier weather, remained very wet, and the threat of downy mildew was season long. Several wine grape vineyards in the area were partially defoliated by the disease and may suffer substantial die-back this winter from poor wood maturity. According to the DMCAST model in VITIS, there were about 40 downy mildew infection periods from late May through September--a few more than in most years. This number is probably an underestimate due to some problems we had this spring with the wetness sensors on our weather station. However, more notable was the difference in average length of the infection periods in different years. The average length of a downy mildew infection period in 2004 was more than double what we had recorded in 01, 02, and 03.

Black Rot:
Obviously the moisture this year generated a similar picture with respect to black rot. Black rot, where present even in trace amounts last year, became a serious problem in 2004 for many growers. Most of the infection periods occurred in July, the worst of which occurred, fortunately, several weeks after bloom. The relatively early bloom for Concord this year allowed time for fruit to have developed some resistance to the disease before the worst of the infection periods hit. Three very extensive infection periods occurred in the second half of July when Concord fruit were relatively resistant. Still, from mid bloom to about 4 weeks after bloom (the bulk of the fruit susceptibility period), it basically rained every other day and we recorded about 9.5 inches of rain during that period; over 2 inches of rain per week. This created conditions for just a trace of black rot to increase to economically damaging levels in some vineyards. To make matters worse, the cooler than average July temperatures likely slowed the development of resistance to the pathogen and may have extended the susceptibility period. These results will provide the potential for serious problems with black rot next year.