SURVIVAL OF XANTHOMONAS AXONOPODIS PV. MANIHOTIS IN WEED SPECIES AND IN CASSAVA DEBRIS : IMPLICATION IN THE EPIDEMIOLOGY OF CASSAVA BACTERIAL BLIGHT.

Weeds from 10 different families were collected weekly from a cassava field previously inoculated with an antibiotic resistant strain of Xanthomonas axonopodis pv. manihotis (Xam) and screened for presence of the pathogen. In the glasshouse, weeds transplanted in pots were infiltrated with a highly virulent marked Xam strain, and the development of the Xam population was quantified. Under field conditions, Xam survived epiphytically for at least 30 days at moderate concentration on Brachiaria deflexa (Poaceae), Mariscus alternifolius (Cyperaceae), Pupalia lappacea (Amaranthaceae) and Solanum nigrum (Solanaceae), whereas lower Xam concentrations were determined on Dactyloctenium aegyptium (Poaceae), Talinum triangulare (Portulacaceae) and Tridax procumbens (Asteraceae) within 30 days. Some other weeds supported a shorter survival time of the pathogen, such as Cyathula prostrata (Amaranthaceae), Digitaria horizontalis (Poaceae), Euphorbia heterophylla (Euphorbiaceae) and Physalis angulata (Solanaceae). After infiltration of leaves with Xam suspension in the glasshouse, Vernonia cinerea (Asteraceae), Dactyloctenium aegyptium and Brachiaria deflexa supported survival of Xam for 60 days. Under field conditions as well as in the glasshouse, none of the weed species developed disease symptoms similar to Cassava Bacterial Blight (CBB), so that, alternative host could not be identified among the weed species studied. However, since the pathogen survived or even multiplied on some weed species, these plants may constitute an inoculum source from which the bacteria can be transferred to cassava plants. The role of infected debris in the survival of Xam in the field was studied by keeping infected cassava leaves on the soil surface, by covering infected cassava leaves with soil, or by burying the infected leaves at 25 cm to 30 cm soil depth. In the glasshouse, debris mixed with soil and unmixed debris with soil were subjected to different water regimes to simulate the seasons. Our investigations proved that the survival period of Xam in debris on the soil surface was negatively correlated with rainfall. During the trial of 1996, the total rainfall was less than the rainfall in 1997, and the survival of Xam in debris on the soil surface reached at least 60 and 45 days, respectively. When the debris were covered with soil or buried in soil, the survival reached only 30 days. The survival reached less than 60 days when debris mixed with the soil were moistened three times a week. Under dry conditions, Xam survived for more than 150 days. When soil was contaminated by a pure bacterial suspension of Xam, the bacteria survived only for three weeks at very low numbers. Thus, infected debris on the soil surface could conserve the pathogen for a long period depending to the environmental conditions and constitute an inoculum source for a new plantation.