Identification of Factors that Influence Screening for Bacterial Blight Resistance

Inoculation of plant pathogenic bacteria in field plots has been improved by the use of organosilicone surfactants. Factors, such as cultivars, number of applications, crop stage, plant density, concentration of bacteria, and irrigation application method (overhead spray versus drop hoses), that may affect field screening of cotton cultivars for resistance to bacterial blight caused by Xanthomonas axonopodis pv. malvaceaum (Xam) were tested. Plants were inoculated with Xam (race 18) plus the organosilicone non-ionic wetting agent, Silowet L-77 (0.25% v/v). Disease incidence (DI) was similar for plants inoculated with Xam 1, 2, and 3 times. DI was not affected by Xam concentration when applied at 106 to 108 cfu/ml. DI was affected by the number of leaves at the time of inoculation for a susceptible cultivar (Paymaster 2326 RR) in 1 of 3 tests. The number of leaves at the time of inoculation had no effect on DI of the susceptible cultivar (PM 2326 RR), a partially resistant cultivar (PM 2200 RR), or an immune cultivar (FiberMax 960BR) in two tests. Plant density ranging from 3 to 16 plants/m row positively affected DI for PM 2326 RR and negatively affected DI for PM 2200 RR, although the effect of plant density was small. Overhead irrigation increased DI in PM 2200RR compared with drop hoses, but irrigation method did not affect DI for PM 2326RR. Based on these results, the suggested protocol for inoculating plants is as follows: 106 cfu/ml Xam plus Silowet L-77 (0.25% v/v) applied in 470 L/ha of water over the top of cotton. Number of leaves (2 to 8), plant density (3 to 16 plants/m row), and irrigation method did not substantially affect the ability to distinguish between susceptible and partially resistant cultivars. Environmental factors had a smaller affect than cultivar (susceptible versus partially resistant) on the development of symptoms.