The Effect of Solute Potential and Water Stress on Black Scorch Caused by Chalara paradoxa and Chalara radicicola on Date Palms.

Date palm trees (Phoenix dactylifera) were found to be infected with Chalara radicicola and Chalara (Thielaviopsis) paradoxa in 1992. Compared with healthy palms, most of the diseased palms appeared to be drought stressed and poorly maintained in landscape settings and nurseries. Water potential studies conducted in growth chambers with 5- to 6-leaf seedling plants subjected to water stress at -2.3 MPa had relatively larger necrotic lesions that developed into cankers, death of buds, and eventually plant death. Tissue necrosis was directly related to water potential. Histological studies showed many necrotic islands of parenchyma tissue in drought-stressed infected plants. Only a few inoculated plants in the growth chamber study died without developing extensive cankers, apparently due to the invasion of the crown or terminal bud by the pathogens. In vitro studies with potato dextrose agar amended with glycerol, NaCl, and KCl to decrease the osmotic matrix-based water potential of the media (-4.25 MPa) resulted in a decrease in the radial growth, biomass, and the sporulation of C. radicicola and T. paradoxa. Solute potential of -0.35 to -1.97 MPa, however, favored the growth of both fungi. Sodium chloride had the greatest effect on the growth characteristics of both fungal species. These studies indicate that in parts of Kuwait where drought and salinity prevail, opportunistic pathogens such as C. radicicola and T. paradoxa could become aggressive and cause serious damage to date palms.