Resistance to bacterial blight of cotton varieties homozygous for combinations of B resistance genes

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Abstract
In the Sudan, inoculation experiments were done on a wide range of Egyptian cotton lines homozygous for single genes and for digenic and trigenic combinations of Knight's B genes for resistance to bacterial blight. Although leaf inoculation was successful, stem inoculation was only partially so and boll inoculation, using two different techniques, failed to produce measurable disease symptoms. There was a good general relationship between leaf and stem resistance, and a close association between resistance to natural attack in the field and leaf resistance to artificial inoculation. Confirmation was obtained of the strong resistance conferred by B2B9K, which was as effective as B2B6 or B2B3B6. No other combination was as effective when inoculated artificially. Nevertheless, in a natural field attack, only mild symptoms were found in lines homozygous for B1B9K, and for B4B6. Under the same conditions lines with B2B6 showed no symptoms but those with B2B3 were severely attacked. Reasons for continuing to use major genes in the breeding of blight-resistant cotton in the Sudan are discussed.