5.1.4
COMPARISON OF FOLIAR DISEASE INTENSITY IN SEEDBEDS OF DIFFERENT ONION VARIETIES AND LOCAL SELECTIONS

V GEPP, I FRIONI, D ROLANDO, P MONDINO and G GALVAN

Facultad de Agronomía, Av. Garzón 780, 12900 Montevideo, Uruguay

Background and objectives
Onions are one of the most important vegetable crops in Uruguay. They are normally transplanted from seedbeds sown in the open. Leaf blight and other diseases caused by Botrytis spp. are common at this stage and cause low quality transplants and even complete loss of plants. About 50% of the onion seed sown in Uruguay is produced on the farm using local selections. Since 1987, the Faculty of Agronomy has been studying these landraces, some of which have shown good agronomic qualities and seem more resistant to diseases. The objectives of this research were to compare local selections and commonly used cultivars in their susceptibility to foliar diseases in the seedbed.

Materials and methods
Five varieties were compared: two of short-day onions, Valencianita and Salto Grande, selected for production in the north of Uruguay and Texas EG 502; two of long-day, Spanish-type onions, Valcatorce INTA, an Argentinean cultivar often planted in Uruguay, and Selección Valenciana, selected in the south of Uruguay; and UR 8818, an intermediate-day landrace collected by the Faculty from the main onion-producing area in the south of the country.

Three seedbeds were sown on March 19, May 3 and June 11, 1993. Each consisted of 22 plots (1x1 m), corresponding to the four replicates per sowing date per variety, plus one extra plot of Valcatorce at each end as a border. No fungicides were applied, and dry onion leaves were spread between the beds to increase the level of inoculum. The plots were arranged at random in March and in randomized blocks in May and July. Readings were taken on 40 plants of the centre of each plot, 70, 90 and 110 days after sowing. Percentage of leaf area covered by spots and incidence of leaf blight were estimated. Each plot was also observed to get a general idea of the amount (low, medium or high) of dead tissue. After surface sterilization, pieces of leaf spots or of blighted tissue were plated out on potato dextrose agar, V8 agar or the selective medium proposed by Kritzman and Netzer [1].

Results and conclusions
Incidence of leaf blight was high in all the materials, ranging from 40-80%. Leaf spot severity varied between 0 and 12%. No material was completely resistant and coefficients of variance were high, but significant differences were encountered in 17 of the 36 evaluations analysed. In all of these, UR 8818 was in the least affected group; in over half of the evaluations it was the best material. Valcatorce was generally in the group with most symptoms. The other varieties were intermediate in susceptibility. These results show the lower susceptibility of the local selection, which makes it recommendable for sustainable production with low fungicide use.

Botrytis sp. was isolated from 18% of the leaf blight pieces but from none of the leaf spots. Stemphylium sp. was the most commonly encountered genus, 32.5% of the leaf blight and 40% from spots. This low isolation rate of Botrytis sp. has been encountered by other researchers. Stemphylium sp. probably colonized tissues weakened by Botrytis sp. or other causes.

References
1. Kritzman G, Netzer D, 1978. Phytoparasitica 6, 3-7.