No abstract

Colonization of Xanthomonas euvesicatoria was investigated in pepper blossoms and the relationship between inoculum concentrations and seed infestation was determined. Inoculation of blossoms resulted in asymptomatic pepper fruit. However, real-time polymerase chain reaction detected X. euvesicatoria in 39% of the seed lots assayed and viable colonies were recovered from 35% of them. Successful transmission occurred in 16% of the seed lots tested. In a separate experiment, X. euvesicatoria reached populations of up to 1 × 10(5) CFU/blossom on stigmas 96 h after inoculation. Bacteria colonized stylar and ovary tissues with populations ranging from 1 × 10(5) to 1 × 10(6) CFU/blossom 96 h after inoculation. A positive correlation existed between inoculum concentration and percentage of infested seedlots. Blossoms inoculated with Acidovorax citrulli also resulted in infested pepper seedlots. Furthermore, A. citrulli colonized pepper blossoms significantly better than X. euvesicatoria by 96 h postinoculation. It was concluded that pepper blossoms can be a potential site of ingress for X. euvesicatoria into seed, and blossom colonization may be involved in pepper seed infestation. Data also indicated that seed infestation via blossoms may be nonspecific because nonhost plants can be colonized by incompatible pathogens. Thus, host-pathogen interactions may not be important for bacterial ingress through blossoms.

Sweet onions (Allium cepa L.) are typically grown on bare soil and irrigated with high-pressure systems such as sprinklers or center-pivots. The objective of this study was to determine the effects of irrigation system and mulch on bolting, bulb yield and bulb quality over 3 years. The experimental design was a split plot, where the main plot was irrigation system (drip or sprinkler) and the subplot was the type of mulch (bare soil, black plastic film or wheat straw). The results showed that individual bulb weight and bulb yields under drip irrigation were similar to those under sprinkler irrigation. Plants grown on bare soil had the highest total yield during the three seasons and among the highest marketable yield. There were no consistent differences in the bulb number or yield of plants on plastic film mulch compared to those of plants on wheat straw mulch. Plants on wheat straw mulch had reduced foliar nitrogen content. Variability in yields among mulches and seasons was partly explained by changes in seasonal root zone temperature and soil water potential. Total and marketable yields and weight of individual bulbs increased with increasing root zone temperatures up to an optimum at 15.8 °C, followed by reductions in yields and individual bulb weight at >15.8 °C. Onion bolting increased with decreasing foliage nitrogen content, with plants on wheat straw having the highest bolting incidence. Bolting also increased with decreasing root zone temperatures for the season. Total and marketable yields increased with decreasing mean seasonal soil water potential down to -30 kPa. Irrigation system and mulches had no consistent effect on the soluble solids content or pungency of onion bulbs.

Thomas, A., Langston, D. B., Jr., Sanders, H. F., and Stevenson, K. L. 2012. Relationship between fungicide sensitivity and control of gummy stem blight of watermelon under field conditions. Plant Dis. 96:1780-1784.Gummy stem blight (GSB), caused by the fungus Didymella bryoniae, is the most destructive disease of watermelon and is managed primarily with fungicides. D. bryoniae has developed resistance to many fungicides that were once very effective, including azoxystrobin, boscalid, and thiophanate-methyl. Field experiments were conducted in Tifton (TN) and Reidsville (RV), GA in 2009 and 2010 to establish a relationship between frequency of resistance to a fungicide based on in vitro assays and its efficacy in the management of GSB. Frequency of resistance to boscalid, thiophanate-methyl, and azoxystrobin was >0.80 in isolates collected from nontreated plots in both locations and years. All isolates collected after six applications of boscalid, thiophanatemethyl, or azoxystrobin were resistant to the respective fungicide. All isolates collected from treated and nontreated plots were sensitive to tebuconazole and difenoconazole. GSB severity was assessed on a weekly basis from 63 days after planting. GSB severity in plots treated with boscalid, thiophanate-methyl, or azoxystrobin was not significantly different from that in the nontreated plots (39%, TN-2009; 45%, TN-2010; and 16%, RV-2010). GSB severity in tebuconazole-treated plots (27%, TN-2009; 14%, TN-2010; and 4%, RV-2010) was significantly lower than all other treatments and the nontreated control. There was a consistent negative association between frequency of fungicide resistance and disease control in the field. Thus, knowledge of the frequency of fungicide resistance in the pathogen population will be helpful in selecting the most effective fungicides for the management of GSB in watermelon fields.

Mulch (black plastic, wheat straw, or bare ground) and irrigation (drip or overhead sprinkler) treatments were evaluated for their effect on center rot of onion (Allium cepa L.), caused by the bacterium Pantoea ananatis, over the course of two seasons. Irrigation type had no effect on center rot incidence or severity in either year. In contrast, center rot development was delayed by 7 to 14 days on onions grown in straw mulch or bare ground compared to those in black plastic. Straw mulch resulted in later harvest dates and was associated with reduced levels of center rot. In contrast, black plastic increased disease incidence and hastened the onset of the epidemic. The spatial distribution of disease incidence in both years indicated the presence of a primary disease gradient. At harvest, infected plants were segregated by treatment and by duration of infection [based on disease ratings taken from the time of first symptom expression (beginning at 110 to 120 days after transplanting and then every 5 to 10 days until harvest)]. Early-vs. late-infected plants had no significant effect on yield (bulb weight). However, symptom expression in terms of the number of days after planting was significantly correlated with a disease severity index. Amount of rot in bulbs from plants displaying their first symptoms only 1 to 2 days before harvest (late-season infection) was not significant from rot levels in control bulbs at harvest. However, at 4 weeks after harvest, onions from plants with late-season infections exhibited significantly more rot in storage compared to the control.