Screening for Resistance to a Third Race of Fusarium Wilt in Tomato.

Abstract

R. Grattidge Department of Primary Industries, Bowen, Queensland, 4805. Fusarium wilt of tomato caused by Fusarium oxysporum Schlecht. f. sp. lycopersici (Sacc.) Snyder & Hansen has been a major problem in the Bowen district in central Queensland for more than 60 years (4). Race 2 was recorded in 1968 (3) and a third race was found in 1978 in small areas on a few farms (2). Spread of the third race has been rapid and by the conclusion of the 1981 season more than 50% of the farms in the district were infested to some extent. The reaction of many commercial cultivars to the third race and the availability of resistance genes in breeding material and Plant lntroduction (PI) lines were unknown. In 1979 screening for resistance to the third race commenced on locally available material as well as lines from the Plant lntroduction Centre, Iowa, U.S.A. Planthouse Experiments Isolates of F. oxysporum previously used to determine the occurrence of the third race (2) were used in screening trials. Cultivars Walter and Flora-Dade with the 11 gene for resistance to race 2 were included in all tests as a check on pathogenicity of the isolates. Conidial suspensions were prepared by lightly scraping the surface of 7-day-old plate cultures covered with sterile distilled water. The resultant suspension was strained through a fine sieve to remove mycelium, and spore counts were made with an haemocytometer. Plants were inoculated by dipping the washed roots of 10-14-day-old seedlings in a conidial suspension (5 x 106/ml) and replanting the seedlings in steamed UC mix C (1). Thirty plants of each line (when available) were assessed after growth for three weeks in a shadehouse with 30% shade and daily temperature ranges of 12-23OC in winter to 20-30°C in summer. Plants free of wilt symptoms or with only slight discoloration in the tap root were regarded as resistant. Cuttings from resistant plants were struck and grown on to produce seed. One hundred and ninety lines from a number of sources were screened (Table 1) and in 49 of these a few plants survived. There were however only 17 lines in which more than 20% of the plants survived. These were Lycopersicon pimpinellifolium (Jusl.) Mill. (PI 79532, 124039, 143524, 143527,2ll838,2ll84O, Bowen Acc. 472); L. glandulosum C.H. Muller (PI 124540, 126434, 126440, 126443, 126448; L. peruvianum (L.) Mill. (PI 128649,247087,365947, Bowen Acc. 471) and L. pimpinellifolium x L. lycopersicum (L.) Krast. ex Farw. (PI 269140). Two of these lines (PI 124039 and Bowen Acc. 472) have been included in a plant improvement programme. The other 15 lines are being further evaluated in planthouse tests. Field Experiment The root inoculation method may be so severe that field resistance is not detected. Commercial cultivars and breeding lines in which either a few plants survived or wilt symptoms developed very slowly were thus planted in an area where race 2 resistant cultivars were severely affected the previous season. The trial included two commercial cultivars (Walter and E.S.58) and 12 breeding lines (S12, S48, S49, S51, US 629, US 638, 72TR, Q97719, PI 124039, Bowen Acc. 493, and two selections of F2s of commercial Hybrid Jackpot). These were planted in 5 plant plots replicated twice.