Evaluation of Solarium ochranthum as a potential source of late blight resistance
Kobayashl, R.S., Deahl, K.L., Stommel, -J.R. and Sinden, S.L.
Vegetable Laboratory, USDA/ARS, Beltsville, MD 20705
Breeding for late blight resistance in tomato has been on-going for more than 45 years. Several cultivars
with monogenic resistance to Phytophthora infestans are available but cultivars with good horizontal (field)
resistance have yet to be developed. A few wild tomato relatives, Lycopersicon pimpinellifolium, L. hirsutum, L.
esculentum var. cerasiforme and L. cheesmanii var. minor, have been identified as possible sources of late blight
resistance (Lukyanenko, 1991). Solanum ochranthum, a self incompatible woody vine-like species found in moist
habitats, belongs to the group of Solanum species that is most closely related to and likely ancestral to
Lycopersicon (Rick, 1988). Rick (1986) suggests that S. ochranthum may be a possible source of resistance to
several fungal diseases. The potential of S. ochranthum as a possible source of late blight resistance was
evaluated.
Three accessions of S. ochranthum, P.I. 230508, P.l. 473498 and P.I. 498268 were obtained from J.
Bamberg (Potato Intro. Stn., Sturgeon Bay, WI). The accession LA 2117 was obtained from C.M. Rick (Univ. Calif.,
Davis, CA). Seeds were pretreated by soaking in 50% household bleach solution for 30 mins., rinsed thoroughly
and germinated on moist filter paper in petri plates prior to planting. Plants were maintained in controlled
environment chambers at 27C under 16 hour daylength. Late blight resistance was measured by a modified
detached leaf test (Turkensteen, 1973). Zoospore inoculum was prepared from Phytophthora infestans isolated
from infected leaves of tomato and potato (Deahl, at al., 1991). S. ochranthum leaves were inoculated by placing
four 50 I droplets of inoculum on the abaxial surface. Leaves were incubated in plastic containers at 100% RH at
25C. Disease ratings were recorded 4 days after inoculation.
Moderate resistance against both the tomato and the potato isolates of P. infestans was detected among
the four S. ochranthum accessions, with P.I 230508 having the highest degree of resistance (Table 1).
Interestingly, all of the S. ochranthum lines appeared to have a slightly greater tolerance to the P. infestans isolated
from tomato than to the isolate from potato. Our results suggest S. ochranthum may be a promising source of
tomato late blight resistance.
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