INIA-CRI Quilamapu, Postal Box 426, Chillan, Chile

Background and objectives
Apple scab is the main disease affecting comercial apple orchards in Vill Region in Chile. In an ordinary growing season, eight to ten fungicide applications are necessary to control this pathogen (Venturia inaequalis). During the past year, because of the El Nino effect, the applications had to be doubled. Accurate information on the development of the pathogen population contributes to more efficient disease management strategies. The objective of this experiment was to compare the production of pseudothecia and asci in three apple cultivars, since cultivar influence on ascal density and pseudothecial productivity has been reported [1].

Materials and methods
At the end of autumn 1997, infected leaves of cultivars Mutsu, Jonagold and Macoun were collected from beneath unsprayed apple trees (Malus x domestica) and maintained in trays covered with a net in the orchard. A randomized-block experimental design with five replicates was employed. Since August, leaves were checked weekly for presence of pseudothecia under a dissection microscope. The number of pseudothecia was expressed per cm2leaf. Six pseudothecia by experimental unit were removed with a fine needle and transferred to a drop of distilled water on a glass slide. Pressure was applied with a coverslip to split the pseudothecia. The number of morphologically mature, immature and empty asci was recorded under the microscope (x40). The results were adjusted and expressed as a percentage of the total maximum production, according to Gadoury and MacHardy [2].

Results and conclusions
On August 18, cv. Mutsu was bearing 600 pseudothecia/cm2leaf, significantly higher (P=0.001) than cvs Jonagold and Macoun, which had around 400 pseudothecia/cm2leaf. Mature asci were observed from August 18 to October 6 in the three cultivars. In the first evaluation, the number of mature asci per pseudothecium was significantly greater (P=0.001) on cv. Jonagold, with 53 asci. Cultivars Mutsu and Macoun bore 20 and 30 mature asci per pseudothecium, respectively. The maximum mature asci production, observed on August 25, was similar in the three cultivars, with 61 asci in Mutsu, 66 in Jonagold and 71 in Macoun, corresponding to 67% of the total maximum asci per pseudothecium in Mutsu and around 80% in the latter two. Immature asci at the same date were 25% of the total maximum in Mutsu, significantly higher (P=0.05) than cvs Jonagold and Macoun, with 18 and 12% immature asci, respectively. In the following 3 weeks, the total production was maintained around 80 asci per pseudothecium in the three cultivars. In the 5th week, on September 22, this production decreased to around 46 asci per pseudothecium. On September 29, cv. Jonagold bore two mature asci per pseudothecium, significantly less (P=0.001) than Mutsu and Macoun, with nine and 19 asci per pseudothecium, respectively. In October 6, asci production was depleted in Jonagold. Mutsu and Macoun bore two and five mature asci per pseudothecium, respectively. Even though there was no substantial difference between cultivars in asci per pseudothecium production, the period in which the asci were formed began and finished earlier in cv. Jonagold. As all leaves were heavily infected, there was no significant difference between cultivars in percentage of leaf area bearing pseudothecia.

1. Smith CA, MacHardy WE, 1992. Phytopathology 82, 247.
2. Gadoury DM, MacHardy WE, 1982. Phytopathology 72, 92-95.