Role of Phoma medicaginis Malbr. and Roum. var. medicaginis in the etiology of root and crown rot of alfalfa

The role of Phoma medicaginis var. medicaginis (P.m.var.m.) as a crown and root rot pathogen of alfalfa was investigated using inoculations to plants grown in the growth chamber and in the greenhouse. The root system of plants grown in test tubes, slant boards and in plastic cones showed symptoms when inoculated with this fungus. Wounds were not required for fungal entry, but extensive rot occurred when roots were wounded. Inoculations caused larger lesions in the lateral roots than those produced on the main roots.;This fungus penetrated directly and caused discoloration and tissue disintegration. Inter- and intracellular penetration facilitated by enzymatic degradation was the mechanism likely involved in penetration. Colonization of the cortex was intercellular. Access to the xylem elements was achieved radially through the cortex. Host responses included suberization of cortical cell walls and occlusion of vessels with pectic substances and wound gum. Cavities in the cortex, resulting from tissue degradation, were associated with later stages of infection. Intracellular hyphae were observed in dead cells.;The fungus was pathogenic to the crowns only when they were wounded. From diseased crowns, this fungus colonized the xylem tissues of the stems and the hypocotyl-taproot-axis. Discolorations were due to occlusions of vessels. Wound periderm was formed surrounding the infected areas of the hypocotyl-taproot-axis. The pathogen penetrated through the stubs causing dieback. Crown rots can likely be initiated by basipetal colonization of stubs suffering from black stem disease. Dry weight of foliage of alfalfa grown in the greenhouse was significantly reduced by infections of the roots, crowns and stubs by this fungus.;Isolates of P.m.var.m. were relatively stable in morphology and pigmentation of the colony but varied more in final diameter of the colony, rate of radial growth and sporulation. The isolate, and the organ and location from which it was obtained were significant sources of variation. All isolates evaluated were pathogenic inducing root rot and significant losses in yield of alfalfa grown in the greenhouse. Differences in virulence among isolates were expressed in the length of lesions induced in plants grown in the growth chamber.