Characterization Of The Verticillium Dahliae Defoliating Pathotypeon Cotton In Central China And Factors Associated With Prevalence Of This Severe Pathotype

Verticillium wilt (VW) of cotton occurred more and more severe in central China, paticularly the defoliating symptoms appeared in the fields. Knowledge on genetic and virulence divercity in Verticillium dahliae is the key for efficient management of VW. A total of341strains of V. dahliae were collected from cotton in central China for studying the genetic diversity of V. dahliae at three different levels. The first level included129strains from52districts in30counties/cities in central China. The second level included86strains of V. dahliae from7fields with different incidence of VW. The third level included126strains of V. dahliae from6plants. Vegetative compatibility grouping (VCG) assessment and PCR-based molecular pathotyping were used to evaluate the genetic diversity of V. dahliae.The results indicated that in the first level, VCG1A and VCG2isolates accounted for94.6and5.4%, respectively; In the second level, VCG1A isolates prevailed (100%) in5cotton fields, in the remaining2fields, two VCGs were identified with the VCG1A isolates accounting for90.9and94%, respectively. In the third level, the VCG1A isolates accounted for100%.Pathogenicity of19isolates of V. dahliae, including10VCG1A isolates and9VCG2isolates, was tested on seedlings of two upland cotton cultivars E Mian24and Yin Rui361. The isolates from two VCGs were virulent on the two cotton cultivars. After two weeks of post-inoculation (25℃), the VCG1A isolates caused defoliation symptoms, whereas the VCG2isolates caused wilt disease, but did not cause any defoliation symptoms. Therefore, the VCG1A is the severe pathotype, whereas the VCG2is the mild pathotype.A study was conducted to determine the level of resistance of12commercial cotton cultivars to infection by the both pathotypes of V. dahliae under growth chamber conditions.(ⅰ) All the12cotton cultivars were susceptible to the defoliating pathotype (D-pathotype) of V. dahliae. All the12cultivars were more susceptible to the D-pathotype isolates than to non-defoliating pathotype (ND-pathotype) isolates. Six of evaluated cultivars (including C111, Xiang Za Mian3, Fu Kang Za3, Xiang Za Mian2, Sheng Za Mian9, Hua Feng Za1) were susceptible to the ND-pathotype isolates of V. dahliae at different levels. Three of evaluated cultivars (E Za Mian19, Fu Kang Za518, E Za Mian4) were moderately susceptible to ND-pathotype isolates. Three of evaluated cultivars (E Za Mian3, Fu Mian2, Feng Mian.03) were resistant to the ND-pathotype isolates;(ⅱ) A resistant related phenotypic reaction to the disease was quantified by using four growth parameters (plant height, leaf weight, stem weight, total shoot weight) measured27days after inoculation. The four growth parameters of plants inoculated D pathotype isolates were lower than that of plants inoculated ND pathotype isolates. The percentage of decrease in leaf weight was found to be the best parameter to indicate cotton resistance.Effect of temperature on conidial germination, mycelial growth and aggressiveness in two VCGs of V. dahliae was studied,(ⅰ) VCG1A/D isolates and VCG2/ND isolates had the same optimum temperature of25℃for mycelial growth on PDA and CDA media. VCG2/ND isolates grew faster than VCG1A/D isolates at20to25℃, but VCG1A/D isolates grew faster than VCG2/ND isolates at30℃;(ⅱ) Conidia of VCG1A/D isolates germinated more rapidly than conidia of VCG2/ND isolates at temperatures of10-33℃;(ⅲ) The VCG1A/D isolates induced defoliation at25/25℃,27/27℃,30/25℃(day/night temperatures) on two cotton cultivars, whereas VCG2/ND pathotype isolates did not induce any defoliation symptoms on the cotton cultivars. Under30/30℃or33/27℃, the leaf symptoms were not visible. However, the VCG1A/D isolates caused higher vascular discoloration index (VDI) on E Mian24than VCG2/ND isolates. All the D-and ND-pathotypes isolates were not pathogenic on Yin Rui361at33/33℃.This study suggests that the VCG1A/D pathotype has become prevalent on cotton in Hubei Province and the surrounding areas. High susceptibility of the cotton cultivars and high aggressiveness of the VCG1A/D pathotype of V. dahliae on cotton under25-33℃might be responsible for the rapid spread and the prevalence of this pathotype in Hubei.