Screening for resistance to Elsinoe ampelina (de Bary) Shear, causal agent of grape anthracnose, in grapevine seedlings is commonly conducted by natural infection over 3 to 4 years in the vineyard. The objective of this research was to develop a greenhouse screening method for selecting grapevine seedlings with resistance to anthracnose. Spores of E. ampelina were obtained from 3- to 4-week-old cultures on potato dextrose agar. Inoculum concentrations ranging from 1.3 × 103 to 1.3 × 107E. ampelina conidia per mL were evaluated and 106 conidia/mL was optimum. The time of incubation of seedlings in a moist chamber after inoculation varied from 24 to 120 hours with 24 to 72 hours resulting in good symptom development. Temperatures in the moist chamber from 16 to 32 °C were evaluated and the most consistent results were obtained at 20 to 28 °C. The most effective method for selecting anthracnose-resistant grape seedlings in the two-to-three true-leaf stage was misting the seedlings with a suspension containing 106 conidia/mL in water and placing the inoculated seedlings in a moist chamber at 24 °C for 48 hours, followed by 8 days on a greenhouse bench. Resistant seedlings from the greenhouse screening (those with <10 foliar lesions) were transplanted into the vineyard and found to be resistant to anthracnose infection under rainy, humid conditions. This greenhouse procedure for selecting grapevine cultivars and breeding lines with resistance to anthracnose is accurate, economical, and labor-saving.
Prevention of the introduction of bacterial fruit blotch of watermelon, caused by Acidovorax avenae subsp. citrulli, into the transplant house or field is the most effective control strategy. Watermelon seedlots currently are screened for A. avenae subsp. citrulli, but other cucurbits, often grown in the same transplant house or field, generally are not as carefully monitored. In 1997 and 1999 field tests, cultivars of watermelon, muskmelon, honeydew melons, acorn squash, butternut squash, yellow squash, zucchini squash, cucumber, and pumpkin were evaluated for foliar and fruit susceptibility to bacterial fruit blotch and for seed transmission of A. avenae subsp. citrulli. The bacterium was introduced into the field on infected watermelon transplants or by misting a bacterial suspension onto fruit of the cucurbits. Foliar and fruit symptoms were more extensive in the watermelon, muskmelon, and honeydew melons than in the other cucurbits. In greenhouse grow-out assays, seed transmission of A. avenae subsp. citrulli was detected in every cucurbit in at least one of the two seasons, even though there were no fruit symptoms in some of them. Thus, any cucurbit crop plant should be considered a potential source for the introduction of A. avenae subsp. citrulli into the transplant house or field.
The accumulation of Pierce’s disease bacterial occlusions in xylem vessels of leaves of susceptible ‘Schuyler’ bunch grapes occurred 2 months earlier than the accumulation in leaf vessels of the more tolerant ‘Carlos’ and ‘Welder’ muscadines. The first appearance of leaf marginal necrosis symptoms occurred when the accumulation of bacterial occlusions was approaching the maximum—the last week of June in the bunch grape, the last week of Aug. in the 2 muscadines. The site of bacterial accumulation was primarily in the leaves of ‘Carlos’, but in both leaves and stems of ‘Schuyler’ and ‘Welder’. Stem dieback symptoms are more common in the latter 2 cultivars.
Seedlings of 22 watermelon [Citrullus lanatus (Thunb.) Matsum. and Nakai] cultivars and two plant introductions were screened in the greenhouse for resistance to the fruit blotch bacterium. There were significant differences in disease severity among cultivars, but no cultivar was immune to the bacterium. In field tests, fruit of 18 commercial cultivars were inoculated individually or became infected naturally from diseased foliage. Cultivars with relatively resistant fruit included `Sugar Baby', `Jubilation', `Mirage', `Calsweet', `Crimson Sweet', `Royal Sweet', and `Sangria'. The more susceptible cultivars generally had a light-colored rind. Cultivar level of resistance to bacterial fruit blotch may not be sufficient under conditions conducive to severe disease development.
Rickettsia-like bacteria, implicated as the casual agent of Pierce’s disease, were easily observed by electron microscopy in the xylem of muscadine grape (Vitis rotundifolia Michx.) with symptoms of Pierce’s disease. The symptoms included severe stunt, marginal leaf burn, and dieback. The bacterium was observed only in the tracheary elements of the xylem.
Differences in levels of tolerance to Pierce’s disease were found in muscadine cultivars and breeding selections. Muscadine cultivars and selections developed at Experiment, GA were generally more tolerant to the disease than those developed at Meridian, MS or Raleigh, NC, however, there were some highly tolerant cultivars from all 3 locations. Cultivars from MS were much more tolerant to the disease than were selections from that location which were never considered acceptable for release to commercial growers.