Bean common bacterial blight: pathogen epiphytic life and effect of irrigation practices

Abstract

In recent years, bean common bacterial blight (CBB) caused by Xanthomonas axonopodis pv. phaseoli (Xap) has caused serious yield losses in several countries. CBB is considered mainly a foliar disease in which symptoms initially appear as small water-soaked spots that then enlarge and become necrotic and usually bordered by a chlorotic zone. Xap epiphytic population community has a critical role in the development of the disease and subsequent epidemics. The epiphytic population of Xap in the field has two major parts; solitary cells (potentially planktonic) and biofilms which are sources for providing and refreshing the solitary cell components. Irrigation type has a significant effect on epiphytic population of Xap. The mean epiphytic population size in the field with an overhead sprinkler irrigation system is significantly higher than populations under furrow irrigation. A significant positive correlation between the epiphytic population size of Xap and disease severity has been reported in both the overhead irrigated (r=0.64) and the furrow irrigated (r= 0.44) fields.

Keywords: Bean common Bacterial blight; Disease severity; Epiphytic life; Irrigation; Xanthomonas axonopodis pv. phaseoli.

Figure 1

High disease severity showing the consequences of bacterial dispersal by overhead sprinkler irrigation, central part of Iran.

Figure 2

Initial water soaked spots on bean pods.

Figure 3

Enlarged necrotic lesions on bean leaves bordered by a chlorotic zone.

Figure 4

Infected symptomless (left) and symptomatic seeds (right).

Figure 5

Bacterial colonies ofXapon Modified NBY; a new developed semi selective medium.

Figure 6

Comparison of NBY and Modified NBY efficiency in isolation ofXanthomonas axonopodispvphaseolifrom symptomless bean leaves when leaves were washed and the same dilution cultured on both media at the same time. Saprophytes grew on common NBY while, failed to grow on Modified NBY.

Figure 7

In furrow irrigation,Xappopulation size (log cfu/cm2 leaf) increased up to just 10 days after the third trifoliate leaf unfolded (V4) following an initial inoculation at the second trifoliate stage while for overhead sprinkler irrigation, the population size increased up to 40 days. Means with different letters are significantly different. The experiments were conducted in two different fields in the same research farm using a randomized complete block design with eight replicates in Arak, Iran in 2005.

Figure 8

The interaction of irrigation system and time on CBB disease severity. With furrow irrigation, the CBB severity index did not change between the R6 (full flowering) and R8 (pod filling) bean growth stages. In contrast, CBB severity significantly increased from the R6 and R8 bean growth stages for the field using an overhead sprinkler irrigation system. The experiments were conducted in two different fields in the same research farm using a randomized complete block design with eight replicates in Arak, Iran in 2005.