CAVITY SPOT OF CARROTS - INTERACTIONS BETWEEN THE HOST AND PATHOGEN, RELATED TO THE CELL-WALL

This study investigates the structural aspects of cavity spot pathogen esis. Different Pythium spp. isolated from infected carrots, apples an d melons were cultured on agar in Petri dishes and used for inoculatio n of uninfected carrots. Only slow-growing Pythium spp. (<15 mm day(-1 )), such as P. violae and P. sulcatum caused cavity spot lesions. It i s suggested that slow-growing species are able to penetrate, albeit sl owly, into the plant tissue for 3 to 4 days before a hypersensitive re action develops. Fast-growing species, however, did not cause lesions. Based on ultrastructural observations, we suggest that the following sequence of events occurs between the plant and the pathogen: The fung us infects the walls and grows for several days, during which time sma ll amounts of wall-degrading enzymes are secreted. Phenylalanine ammon ia lyase (PAL) activity and phenols increase linearly immediately upon inoculation. There was a lag phase of about 5 days before lignin bega n to increase linearly for about a month. Dissolution of wall componen ts decreases the solute potential and water potential in the apoplast. Thus, water moves from the symplast into the apoplast, the turgor pre ssure gradually dissipates, and the cells shrink and eventually die.