Studies on the bacterial diseases of Sudan crops

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Abstract
SUMMARY: New records are presented of bacterial diseases affecting the following weed plants: Heliotropium sudanicum, Rhynchosia memnonia, Vigna radiata, V. pubigera, Euphorbia acalyphoides and Phyllanthus niruri. Bacterial leaf blight of pigeon pea (Cajanus cajan) caused by Xanthomonas phaseoli f. cajani is reported for the first time from the Sudan. The causal bacteria, all of which belong to the genus Xanthomonas, are compared with related pathogens. The differences between these bacteria are considered of little diagnostic value. Pathogenically, they can be separated into three groups: (i) the Heliotropium pathogen; (ii) the bacteria affecting leguminous plants; and (iii) the bacteria affecting species of the family Euphorbiaceae. Because of the diversity in opinion on speciation in the genus Xanthomonas, two alternative proposals are put forward. In the first, Xanthomonas is considered a monotypic genus with X. campestris as the sole species. The pathogens reported in this work from previously unrecorded hosts, which proved to differ in pathogenicity from established ones, are considered to be new formae speciales. These are X. campestris f.sp. heliotropii from H. sudanicum, X. campestris f.sp. rhynchosiae from R. memnonia, X. campestris f.sp. vigna‐radiatae from V. radiata, X. campestris f.sp. euphorbiae from E. acalyphoides and X. campestris f.sp. phyllanthii from P. niruri. The organism from V. pubigera is accordingly identified as X. campestris f.sp. vignicola. Other bacteria used for comparison are also reduced to ff.sp. of X. campestris.The other proposal is to lump together pathogens with overlapping host ranges in a single species that comprises a number of ff.sp. differing in pathogenicity. Thus the bacteria from leguminous plants are all considered to belong to X. phaseoli as previously suggested by Sabet. New ff.sp. are created for the pathogens from R. memnonia (X. phaseoli f.sp. rhynchosiae), and V. radiata (X. phaseoli f.sp. vigna‐radiatae). The organism from V. pubigera is identified in this case as X. phaseoli f.sp. vignicola. The bacteria from members of the Euphorbiaceae are considered to belong to X. ricini on priority grounds. X. cassavae and X. poinsettiaecola which have been used for comparison are reduced to the ff.sp. X. ricini f.sp. cassavae and f.sp. poinsettiaecola respectively. Two new ff.sp. are created: for E. acalyphoides pathogen, X. ricini f.sp. euphorbiae and for P. niruri pathogen, X. ricini f.sp. phyllanthii.The creation of a new species (X. heliotropii sp.nov.) is proposed for the Heliotropium pathogen.The present study shows that many weed plants may be affected with new bacterial strains that can also infect cultivated crops.