Cheng-Hua Huang

Effect of Application Frequency and Reduced Rates of Acibenzolar-S-Methyl on the Field Efficacy of Induced Resistance Against Bacterial Spot on Tomato

Acibenzolar-S-methyl (ASM), a plant activator known to induce systemic acquired resistance, has demonstrated an ability to manage a number of plant diseases, including bacterial spot on tomato caused by four distinct Xanthomonas spp. The aim of this study was to evaluate application rate and frequency of ASM in order to optimize field efficacy against bacterial spot in Florida, while minimizing its impact on marketable yields. ASM was applied biweekly (once every 2 weeks) as a foliar spray at a constant concentration of 12.9, 64.5, and 129 μM throughout four field experiments during 2007-08. A standard copper program and an untreated control were also included. Overall, biweekly applications of ASM did not significantly reduce disease development or the final disease severity of bacterial spot compared with the copper-mancozeb standard or the untreated control. Only one experiment showed a significant reduction in the final disease severity on plants treated with ASM at 129 μM compared with the untreated control. Three additional field trials conducted during 2009-10 to evaluate the effects of weekly and biweekly applications of ASM at concentrations of 30.3 to 200 μM found that weekly applications provided significantly better disease control than biweekly applications. The tomato yields were not statistically improved with the use of ASM relative to the untreated control and standard copper program. Weekly ASM applications at rates as low as 75 μM (equivalent to 1.58 g a.i./ha in 100 liters of water or 0.21 oz. a.i./acre in 100 gallons of water) to 200 μM (equivalent to 4.20 g a.i./ha in 100 liters of water or 0.56 oz. a.i./acre in 100 gallons of water) were statistically equivalent in managing bacterial spot of tomato without significantly reducing yield compared with the untreated control.

Evaluation of Microbial Products for Management of Powdery Mildew on Summer Squash and Cantaloupe in Florida

Powdery mildew, caused by Podosphaera xanthii (syn. Sphaerotheca fuliginea auct. p.p.), is a serious and frequently occurring disease of cucurbits worldwide. Efficacy of four microbial products (i.e., Actinovate AG, Companion, BU EXP 1216C, and BU EXP 1216S), which contain microbes as the active ingredient, were evaluated on summer squash and cantaloupe against powdery mildew when applied alone or in alternation with a half-rate of conventional fungicide under greenhouse and field conditions. In greenhouse experiments, the product BU EXP 1216S significantly (P < 0.05) reduced the disease severity by nearly 70% relative to the water control. The level of control achieved was not significantly different from that obtained with Procure 480SC (triflumizole), the half-rate of conventional fungicide treatment, in two of four greenhouse experiments. Compared with the untreated water control, BU EXP 1216C and BU EXP 1216S, when applied alternately with Procure 480SC, consistently promoted plant growth measured by plant height, stem caliper, total fresh weight, and chlorophyll content in the leaves. The degree of increase was 11.6 and 11.3% in plant height, 15.6 and 19.8% in stem caliper, 25 and 40.7% in chlorophyll content, and 164 and 250% in total fresh weight, respectively. Alternating applications of these products with Procure 480SC resulted in significantly less powdery mildew disease than in the water control. In the first field trial on summer squash, all products applied individually or in alternation with Procure 480SC significantly reduced the severity of powdery mildew at the early stage (60 days after planting [DAP]) of disease development. Moreover, these alternating treatments resulted in significantly better control than with Procure 480SC alone at the late assessment stage (88 DAP). The products in alternation with Procure 480SC had a level of disease reduction equivalent to Procure 480SC alone on cantaloupe and significantly reduced disease severity in comparison with the water control. Compared with applying the microbial products alone, alternating applications of these products with Procure 480SC significantly reduced disease severity on cantaloupe and improved the marketable fruit number and weight. The data from our studies suggest that these microbial products could be effectively incorporated into disease management programs. In particular, these microbial products could be integrated into the management of powdery mildew on summer squash and cantaloupe in Florida by alternating their application with low rates of conventional fungicides, potentially reducing the development of fungicide resistance in the pathogen population.

A Novel Xanthomonas sp. Causes Bacterial Spot of Rose (Rosa spp.)

A bacterial spot of rose (Rosa spp.) caused by a xanthomonad was observed in Florida and Texas. Ten representative strains collected from the two states between 2004 and 2010 were used to determine the taxonomic position of this rose pathogen. Fatty acid methyl ester analysis was performed and a nearly 2-kb 16S-23S rRNA intergenic spacer along with flanking portions of the 16S and 23S rRNA genes were sequenced for selected strains, showing that they were members of the genus Xanthomonas. Multilocus sequence typing and analysis (MLST/MLSA) and pathogenicity tests were conducted to further characterize the Xanthomonas strains. The MLSA, based on six gene fragments-fusA, gapA, gltA, gyrB, lacF, and lepA-showed that the rose strains fell into Xanthomonas axonopodis subgroup 9.2 and shared the highest similarity values (98.8 to 99.7%) with X. alfalfae subsp. citrumelonis of the subgroup. However, principal coordinate analysis grouped the rose strains into a unique cluster distinct from other members of the subgroup according to virulence phenotypes on 11 plant species belonging to five plant families (Araceae, Euphorbiaceae, Rosaceae, Rutaceae, and Solanaceae). Moreover, the rose strains were aggressive on rose and Indian Hawthorn (Rhaphiolepsis indica). On the basis of the MLSA and virulence phenotypes, the pathovar epithet rosa is proposed.

Management of Powdery Mildew in Squash by Plant and Alga Extract Biopesticides

Although many fungicides are registered for use to control powdery mildew on cucurbits, management of resistance to fungicides in pathogen populations still remains a major challenge. Two biopesticides Regalia SC and HMO 736 were evaluated in the greenhouse and field for their efficacy against powdery mildew in squash. In greenhouses, Regalia SC alone significantly (P < 0.05) reduced powdery mildew compared to the nontreated control, and was as effective as the chemical standard Procure 480SC (triflumizole). In alternation with Procure 480SC, Regalia SC demonstrated greater or equivalent effects on reducing the disease. HMO 736 alone showed varying levels of disease control, but alternating with Procure 480SC significantly improved control efficacy. In addition, application of Regalia SC or HMO 736 each in alternation with Procure 480SC significantly increased the chlorophyll content in leaves and the total fresh weight of squash plants, when compared with the water control, Regalia SC and HMO 736 alone. In field trials, application of Regalia SC and HMO 736 each alone significantly reduced disease severity in one of two field trials during the early stage of disease development, but not during later stages when disease pressure became high. Both Regalia SC and HMO 736 each applied in alternation with Procure 480SC significantly improved the control efficacy compared to Procure 480SC alone. Results from this study demonstrated that an integrated management program can be developed for powdery mildew in squash by integrating the biopesticides Regalia SC, HMO 736 with the chemical fungicide Procure 480SC.