Effect of Temperature on the Formation of Microsclerotia of Verticillium dahliae

Microsclerotium formation by six isolates of Verticillium dahliae was studied at di€erent temperatures both in vitro and in Arabidopsis thaliana. In vitro mycelial growth was optimal at 25°C, but microsclerotium formation was greatest at 20°C (two isolates) or 15±20°C (one isolate). Seedlings of A. thaliana were root-dipped in a conidial suspension, planted, and either placed at 5, 10, 15, or 25°C, or left at 20°C until the onset of senescence, after which some of the plants were placed at 5, 10, 15, or 25°C. The amount of microsclerotia per unit of shoot weight was assessed in relation to isolate and temperature. The optimal temperature for production of microsclerotia was 15± 25°C. Two isolates each produced about 10 times more microsclerotia than each of the other four isolates. For these isolates, high R2 adj.-values of 0.77 and 0.66 were obtained, with temperature and its square as highly signi®cant (P < 0.001) independent variables. R2 adj.-values for the other isolates varied between 0.28 and 0.39. Moving plants to di€erent temperatures at the onset of senescence led to microsclerotial densities that were intermediate between densities on plants that had grown at constantly 20°C and plants grown at other temperatures. This suggests that vascular colon- ization rate and rate of microsclerotium formation are similarly a€ected by temperature. The senescence rate of plants appeared unimportant except for plants grown at 25°C, which showed the highest amounts of microsclerotia per unit of plant weight in the most rapidly senescing plants.