6.67
INOCULUM SOURCES OF XANTHOMONAS CAMPESTRIS PV. VIGNICOLA, INCITANT OF COWPEA BACTERIAL BLIGHT AND PUSTULE, AND IDENTIFICATION OF HOSTS OTHER THAN VIGNA UNGUICULATA

R SIKIROU1, K WYDRA1 and K RUDOLPH2

1International Institute of Tropical Agriculture, PHMD, Cotonou, Benin; 2Institute of Plant Pathology and Plant Protection of the University of Gottingen, Germany

Background and objectives
Cowpea bacterial blight caused by Xanthomonas campestris pv. vignicola (Xcv) is widespread in most areas where cowpea (Vigna unguiculata) is cultivated. It causes heavy losses when the infection enters at an early stage of crop growth. Losses range from 2.66 to 92.24% according to cultivar and stage of infection [1]. Besides the use of resistant varieties [2], understanding of inoculum sources is important for the development of control methods. The objectives of this study were to find out the major inoculum sources of cowpea bacterial blight, and to determine other possible hosts of Xcv besides Vigna unguiculata.

Materials and methods
In the field, cowpea leaves infected with a marker strain resistant to rifampicin and streptomycin were mixed with soil and kept on the soil surface or buried at 10 and 20 cm in the soil. In the greenhouse, 40 g of leaves with symptoms were mixed with soil and subjected to four different water regimes: pots without watering, pots watered every 2 weeks, pots watered every 3 days for 2 months, pots watered every 3 days. Residual bacteria were determined every 2 weeks in field trials and every month in the greenhouse trials.

In field experiments, unweeded cowpea plots were inoculated by spraying with a marker strain resistant to rifampicin and streptomycin. Leaves of the dominant weeds were collected every 5 days for detection of Xcv bacteria. The same weed species were inoculated by leaf infiltration in the greenhouse and leaves were collected for determination of bacterial numbers every 2 weeks. Twelve legume species were tested by leaf infiltration to identify the host range of Xcv. Symptom development was recorded every 3-5 days.

Results and conclusions
The results showed that in the field, no bacteria could survive more than 2 months in the top soil layer or buried at a depth of 10 or 20 cm in the soil. In the greenhouse, the bacteria were also detected only up to 2 months under various water regimes. The epiphytic bacteria obtained from six weed species collected from the field and from leaf-infiltrated plants from the greenhouse progressively decreased in number. In the field, bacteria were not found from 10, 15, 25 and 45 days post-inoculation (d.pi.) and in the greenhouse from 45, 90 and 105 d.p.i., according to weed species. From 12 legume species leaf-infiltrated with Xcv, only Sphenostylis stenocarpa (African yam bean) showed clear and typical symptoms of cowpea bacterial blight.

According to the fast-decreasing pathogen numbers on weeds and the short survival of Xcv in crop residues in the soil, weeds and plant debris are not regarded as principal sources of inoculum, but may contribute to the spread of Xcv during the cropping season. However, Sphenostylis stenocarpa can be considered as a host plant of Xcv.

References
1. Kishun R, 1989. Indian Phytopathology 42, 241-246.
2. Wydra K, Fanou A, Sikirou R et al., 1997. Phytomedizin 27, 45-46.