Vegetative Compatibility Groups in Verticillium Dahliae: Virulence to Cotton

A. A. Bell


 
ABSTRACT

Mutants deficient for assimilation of nitrate nitrogen were used to determent the vegetative compatibility of 89 isolates of Verticillium dahliae obtained from 31 different states and countries and from 35 different crops. Spontaneous mutants were selected as rapid growing sectors on media amended with 1.5 or 2.0 % sodium or potassium chlorate or on minimal medium with only hypoxanthine as a nitrogen source. Nine different complementing groups were found among mutants from single isolates of the fungus. These correspond in behavior to genes for the structural protein of nitrate reductase (nit-1), a nitrogen metabolite control protein (nit-2), a nitrate induction control protein (nit-3), a nitrate uptake (permease) protein (nit-4), and five proteins involved in the synthesis of the molybdenum cofactor of nitrate reductase (nit-5) to -9 grow as thin hyaline colonies on media containing only nitrate as a nitrogen source. Colonies of the mutants have the same linear growth rate as the wild parent, but they grow largely submerged in the medium and produced greatly reduced numbers of conidia and almost no microsclerotia. Mutants for nit-3 also were hyaline initially but eventually formed more conidia and microsclerotia than the nit-1 mutants.

The nit-1 or -3 mutants strongly complemented the mutants for molybdenum cofactor synthesis (nit-5 to -9), and such pairings were used to establish vegetative compatibility between strains. When two strains with the different mutations were compatible and their hyphae fused at the interface of colonies placed about 1 cm apart on minimal medium containing only nitrate nitrogen, the resulting heterokaryon formed abundant aerial mycelia, conidia, and microsclerotia along the line of the interface. In the absence of compatibility, i.e., hyphal fusion, no surge of normal growth occurred at the point of contact of dissimilar mutants.

Pairing among nit-1 or -3 and nit-5, -6, -7, -8, or -9 mutants from the 89 isolates of V. dahliae showed that 84 isolates belonged to one of three vegetative compatibility groups designated as VCG-1, -2, and -4. The remaining five isolates, all from Russia, did not complement any of the other 84 isolates. Thus, they probably represent a fourth compatibility group. Each VCG group could be further divided into A and B subgroups based of the vigor on complementation with various tester strains within the VCG.

Isolates in the VCG-1A subgroup mostly originated from Mississippi to California in the southern USA and form Northern Mexico indicating that this population may be indigenous to this geographical area. The VCG-1B isolates originated from the North Central USA and Canada. The VCG-4 isolates originated form the Northern USA (Washington and Oregon to Wisconsin), Canada, South Australia, and New South Sales. The VCG-2 isolates had a worldwide distribution, and only isolates of the VCG were found in areas such as Europe, Asia, and Africa. Thus, this may be the most primitive of the four VCG groups.

Most of the 20 isolates of V. dahliae obtained from cotton in the USA and Mexico belonged to the VCG-1A subgroup, whereas the nine from Europe, Asia, and Africa belonged to the VCG-2 group and the two from Australia was from the VCG-4B subgroup. One isolate from Peru was from the VCG-1A subgroup, whereas two others and on from Argentina were from the VCG-2 group. Isolates from the VCG-2 group have been obtained from cotton in several fields in California and Texas, and a single isolate from VCG-4A was obtained from naturally infected cotton grown in soil from Temple, Texas in the greenhouse at College Station, Texas. Thus, different genetic types of V. dahliae may be responsible for cotton wilt depending on the geographical area.

All isolated in the VCG-1A subgroup, regardless of host or geographical origin, completely defoliated 'Rowden' cotton at a mean temperature of 25 C. None of the isolates in the other VCG groups of subgroups caused complete defoliation. The next most virulent isolates were from the VCG-2A subgroup followed by the VCG-1B and the VCG-2B subgroups. With two exceptions, isolates from the VCG-4 group caused only mild to moderate symptoms even in susceptible cotton cultivars. Only isolates from the VCG-1A subgroup caused appreciable defoliation of resistant Acala and Gossypium barbadense cultivars. Isolates from the VCG-1A population are by far the most virulent to cotton and should be used in experiments to resistant cultivars, in the USA.



Reprinted from 1992 Proceedings Beltwide Cotton Conferences pg. 195
©National Cotton Council, Memphis TN

[Main TOC] | [TOC] | [TOC by Section] | [Search] | [Help]
Previous Page [Previous] [Next] Next Page
 
Document last modified Sunday, Dec 6 1998