In the present study we screened the progeny of Vitis vinifera × V. romanetii populations segregating for resistance to powdery mildew and determined the presence of a single, dominant locus, Ren4, conferring rapid and extreme resistance to the grapevine powdery mildew fungus Erysiphe necator. In each of nine Ren4 pseudo-backcross 2 (pBC(2)) and pBC(3) populations (1,030 progeny), resistance fit a 1:1 segregation ratio and overall segregated as 543 resistant progeny to 487 susceptible. In full-sib progeny, microscopic observations revealed the reduction of penetration success rate (as indicated by the emergence of secondary hyphae) from 86% in susceptible progeny to below 10% in resistant progeny. Similarly, extreme differences were seen macroscopically. Ratings for Ren4 pBC(2) population 03-3004 screened using natural infection in a California vineyard and greenhouse and using artificial inoculation of an aggressive New York isolate were fully consistent among all three pathogen sources and environments. From 2006 to 2010, Ren4 pBC(2) and pBC(3) vines were continuously screened in California and New York (in the center of diversity for E. necator), and no sporulating colonies were observed. For population 03-3004, severity ratings on leaves, shoots, berries, and rachises were highly correlated (R(2) = 0.875 to 0.996) in the vineyard. Together, these data document a powdery mildew resistance mechanism not previously described in the Vitaceae or elsewhere, in which a dominantly inherited resistance prevents hyphal emergence and is non-race-specific and tissue-independent. In addition to its role in breeding for durable resistance, Ren4 may provide mechanistic insights into the early events that enable powdery mildew infection.
Table grapes are routinely fumigated with sulfur dioxide (S02) at 5,000 to 10,000 ppm for 20 to 30 min at ambient temperature or during cooling to control postharvest decay in storage. This translates to CT product exposure levels (defined below) of about 800 to 1,500 ppm-h. The grapes are refumigated weekly during cold storage with 1,000 to 5,000 ppm S02. We conducted preliminary experiments in the laboratory to determine if S02 gas had insecticidal activity against a lepidopterous pest of grapes. Thirdinstar OLR larvae were exposed to S02 gas in a 0.3 m3 battery jar. The jar was sealed with a metal lid fitted with a rubber gasket. The lid had six holes drilled in the center, each stoppered with a no. 6 rubber stopper. Sulfur dioxide was introduced from a primary standard grade lecture bottle (99.5% AI) using a constant-flow system (flow rate = 100 ml/min). Sulfur dioxide concentrations were continuously monitored, using an Horiba° infrared analyzer with a 200 mm path cell, and recorded every 2 min. Doses were expressed as CT products (ppm-h): the product of concentration (ppm) and time of exposure (h). Tests were conducted at 22 ° 2°C and 90 to 95% relative humidity. Third-instar larvae were placed inside cylindrical cages (2 cm x 6 cm) made from 50 mesh stainless steel wire (n = 14 to 20 larvae per cage). Five cages, each attached to the bottom side of a no. 6 rubber stopper, were suspended inside that jar through the holes in the lid. Every 12 minutes, one cage was removed and the hole quickly plugged with another stopper. By this method, we obtained exposure times of 0.2, 0.4, 0.6, 0.8, and 1.0 h with only minimal effect on the concentration of the S02 gas inside the chamber. One cage was left unexposed (control) outside the test area. Following the test, both fumigated and control larvae were transferred to an agar-based semi-synthetic diet in 30 ml plastic cups (1 larva per cup) and held at 27 ° 1°C. Mortality evaluations were made at the time of transfer to the diet cups and at 1, 2, and 4 days posttreatment. The test was replicated three times.
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