Twelve Pythium isolates were obtained from lettuce roots grown hydroponically in commercial systems, showing or not symptoms of rotting. Three of them were identified as P. helicoides (H1, H2 and H3), whereas five were shown to belong to group F (F1-F5) and four to group T (T1-T4) of Pythium. The identification of the species was based on morphological characteristics. The effect of temperature (10, 15, 18, 21, 24, 27, 30, 37 and 40ºC) on the mycelial growth was determined for all isolates. Minimum and maximum temperatures, estimated by the generalized beta function, varied from 3.5 to 10ºC and 40 to 40.7ºC, respectively. The optimum temperature ranged from 24 to 37ºC for P. helicoides, from 25 to 35ºC for isolate F4 and 21 to 30ºC for the remaining isolates. Pathogenicity and aggressiveness of the isolates were evaluated by the inoculation of lettuce seeds plated in water-agar, at 21 and 30ºC. At 30ºC, P. helicoides isolates were clearly the most aggressives, determining 100 % seed mortality soon after germination. At 21ºC, all isolates reduced seedling growth, associated or not with root tissue necrosis. This is the first report of P. helicoides in Brazil and the first world reference of this species in hydroponic systems.
Lactuca sativa L.; Pythium helicoides; Pythium group F; Pythium group T