Research was carried out to evaluate the effectiveness of the biological control of two most important fungal diseases of lettuce (Lactuca sativa L.): 1) Botrytis Gray Mould caused by Botrytis cinerea Pers. ex Fr.; 2) Sclerotinia Drop caused by two pathogenic fungi, Sclerotinia sclerotiorum (Lib.) De Bary and/or Sclerotinia minorJagger. Biological control in lettuce was carried out: 1) using Coniothyrium minitans Campbell, an antagonist fungus that attacks and destroys sclerotia within the

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... 2) identifying lettuce genotypes showing less susceptibility or tolerance. The object of this research was to find control strategies to reduce chemical treatments. The use of resistant varieties is one of the most economical ways to control vegeTable diseases. The lettuce genotypes showing in preliminary trials the best behaviour to the sclerotial diseases were compared in a randomized complete block experiment design and replicated four times. Observations were carried out from February up to April registering the number of diseased plants and yield. The pathogens were isolated on PDA medium and identified. The isolates grown onto PDA plates, after incubation for 6 weeks, allowed obtaining sclerotia that were the target of C. minitans in biological control trials. In laboratory, in controlled conditions, 27 small plots (30 cm in diameter each) with disinfected soil were performed. In 18 plots 9 sclerotia were inoculated (per plot, three of each fungus) and in 9 plots of them a suspension of the antagonist fungus was added. Subsequently, three lettuce varieties were transplanted. For each variety were compared: 1) untreated plots; 2) treated plots with sclerotia only; 3) treated plots with sclerotia and C. minitans suspension. The number of diseased plants was recorded. According to symptom evaluation scale, ranged from 0 (no disease) up to 10 (100% necrotic leaves or dead plants) the plants were grouped into infection classes, calculating the McKinney index. In greenhouse trials, "LM 1307" genotype showed less significant susceptibility to Botrytis Gray Mould (0-2% of affected plants), while "Ninja" and "Charmy" showed 4-11% and 16-26% of diseased plants, respectively. The yields were 69.7, 62.7, 55.3 t/Ha, respectively. In laboratory tests, the McKinney index gave the following results: no disease in all untreated plants; 38.3, 54.2 and 89.2% in "LM 1307", "Ninja" and "Charmy" treated with sclerotia only, respectively; 2.5, 7.5 and 20.8% in "LM 1307", "Ninja" and "Charmy" treated with sclerotia and C. minitans, respectively. In conclusion, the less susceptibility of the genotypes to sclerotial diseases and the use of hyperparasites of sclerotia of phytopathogenic fungi exhibited best results.