Penetration and infection of alfalfa roots by Phytophthora megasperma and the pathological anatomy of infected roots
Penetration of the fine roots of Vernal alfalfa by Phytophthora megasperma was investigated in solution culture. The zoospores encysted on the root tips around the zone of cell division and cell extension and directly penetrated the host by a small infection peg. No appressoria were observed, but the flattened base of the cyst adhered to the host and appeared to serve the same function. Inside the root the hyphae were inter- and intra-cellular. In naturally infested and artificially inoculated field soils most of the fine roots were destroyed and large lesions appeared on the tap root. The latter originated from infection of the spongy-phellem cells formed around the base of the fine lateral roots. Restricted lesion development on the tap root was associated with the formation of wound periderm around the infection court. No wound periderm was formed when the vascular system was infected and only minor damage occurred when invasion was restricted to the cortex. Root damage reduced growth rates in older plants and severe infection produced yellow foliage, premature defoliation, and wilting.