Determination of Verticillium Dahliae (T-1 Strain) Inoculum in Partially Decomposed Cotton Tissue in Relation to the Total Inoculum in Field Soil

W.C. Schnathorst


 
ABSTRACT

The determination of inoculum density (also often referred to as inoculum potential) of Verticillium dahlias Kleb. in infested cotton field oils largely ignores the inoculum contributed by infected plant tissue. Use of the Andersen Sampler, soil assay technique prevents the deposition of tissue fragments larger than 1 mm across on assay plates. The study reported here attempts to address the question of how much inoculum is present in infected, partially decomposed cotton debris, and the extent of its contribution to the total inoculum in field soils. In addition, if infested soil is milled prior to assay, many propagules originally imbedded in tissue would be released possibly leading to extremely elevated and misleading inoculum density determinations.

Three samples of air-dried infested soil, collected at a depth of 15 cm in winter from a cotton field where verticillium wilt caused by the T-1 strain was epidemic, were weighed. Cotton tissue was removed by dry sieving through a 1mm mesh screen and also picked by hand with a forceps. The plant tissue was weighed separately, milled, and also ground with no--tar and pestle. Ten-mg tissue samples (10 replication for each soil sample) were assayed for V. dahliae propagules with an Andersen Sampler. The soil fraction was assayed in the same manner. Water agar plus dihydrostreptomycin sulfate, chlortetracycline HCl, and chloramphenicol (50 ppm each) was used in assay plates. V. dahliae colonies were counted with a dissecting microscope after 7 and 14 days incubation at 24 C.

In the soil fraction, V. dahliae averaged 304 propagules/g. The cotton tissue fraction comprised approximately 0.5% by weight of the soil samples and averaged 2,850 propagules/g of tissue. Although the tissue fraction of the soil samples contained many more times the number of propagules on an equal weight basis, it was calculated that the tissue actually contributes only 14 additional propagules/g: to the field soil samples. These results indicate that inoculum, density determinations that exclude the fraction are slightly low for total inoculum estimates, but not low enough to justify using a major correction factor in those estimates.



Reprinted from 1985 Proceedings: Beltwide Cotton Production Research Conferences pg. 28
©National Cotton Council, Memphis TN

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Document last modified Sunday, Dec 6 1998