Screening Tomato Seedlings for Multiple Disease

Two procedures for screening tomato (Lycopersicon esculentum Mill.) seedlings for resistance to three pathogens were developed. In one scheme, seeds were sprayed with a spore suspension of Fusarium oxysporum f. sp. radicis-lycopersici Jarvis & Shoemaker (fusarium crown and root rot). Resistant seedlings were root-dipped 2.5 weeks later in a spore suspension of Verticillium dahliae Kleb. (verticillium wilt), and 1 week following the root dip, leaves were rubbed with tobacco mosaic virus. In the other scheme, 2-week-old seedlings were dipped in a spore suspension of F. oxysporum Schlecht f. sp. lycopersici (Sacc.) Snyd. & Hans. races 1 and 2 (fusarium wilt). Resistant seedlings were root-drenched 1.5 weeks later with a suspension of Meloidogyne incognita Kofoid & White (rootknot nematode), and 1 week following, the leaves were rubbed with tobacco mosaic virus. These procedures were effective for disease screening, and their use should reduce the time required for development of two multiple disease-resistant populations. Inbreds from each population could be crossed to produce hybrids resistant to five pathogens. Hybrid cultivars are preferred in the greenhouse tomato in- dustry, in part because resistances to several major diseases of these tomatoes are conferred by single dominant genes that can be combined in hybrids by using inbreds possessing the domi- nant genes in question. Some of the greenhouse tomato diseases for which a single dominant resistance gene is available are: fusarium wilt (Fol) (Bohn and Tucker, 1940; Stall and Walter, 1965); fusarium crown and root rot (Forl) (Berry and Oakes, 1987); verticillium wilt (Verd) (Schaible et al., 1951); rootknot nematode (Meli) (Gilbert and McGuire, 1955); and tobacco mosaic virus (TMV) (Alexander, 1963). The time required to introgress each resistance gene into com- mercially acceptable cultivars is a major restriction. Seedling screening techniques for resistance to all the above diseases are now available. If, however, each technique were used alone, it would take 4 to 5 years to combine all resistance genes into one line. This problem can be alleviated by screening single seed- lings simultaneously for resistance to several diseases, thus sig- nificantly reducing germplasm development time. The objective of this research was to determine the feasibility of screening seedling populations for resistance to three patho- gens. In this study we modified and combined established tech- niques for screening for resistance to Forl (Berry and Oakes, 1987), Verd (Ben-Yephet and Pilowsky, 1979), TMV (Cic- carese and Cirulli, 1980), Fol (Wellman, 1939), and Meli, and then developed procedures to screen single seedlings for resis- tance to Forl, Verd, and TMV, or to Fol races 1 and 2, Meli, and TMV.