Greenhouse techniques to screen for resistance to peanut diseases

Soil-borne diseases have become a significant concern in West Texas peanut production. Cylindrocladium parasiticum is an important peanut (Arachis hypogaea L.) pathogen in the southeastern U.S. and was first confirmed in Texas in 2004. Two experiments were conducted to refine a screening technique for C. parasiticum using peanut germplasm with known resistance levels. Peanuts were screened using two container sizes (66 and 164 cm3), two inoculum densities (15 and 25 microsclerotia (ms)/g soil), and were sampled at three different times (4, 5, and 6 weeks after planting). Fresh root and shoot weights, root rot, and percentage visible taproot and secondary root necrosis were estimated at the three sampling intervals.

Root weight and shoot weights complemented visual root ratings and were the only combinable measurements due to changes in C. parasiticum inoculum that occurred between trials. Percent taproot and secondary root necrosis on highly resistant genotype NC 3033 were significantly lower than on susceptible cultivar NC 7 in 66 cm3 containers at both inoculum densities. Percent taproot and secondary necrosis were simple, quick assessments of disease severity that unlike root rot ratings were continuous and less subjective. Neither taproot necrosis nor secondary root necrosis interacted with the effect of week of sampling which indicated that genotypes differences were apparent at 4 weeks after planting.

In the growth chamber screening test, selected spanish genotypes had lower percentage taproot necrosis than selected runner genotypes (P < 0.0001). The majority of Texas Tech University, Jeffrey Wilson, May 2008 the spanish lines tested in this experiment had mean percent taproot necrosis values equivalent to highly resistant NC 3033. Six runner genotypes also had mean percent taproot necrosis values equivalent to highly resistant NC 3033. Sclerotinia minor is a serious disease of peanuts (Arachis hypogaea L.) in the southeastern U.S. that has become a problem in numerous West Texas peanut fields since 1996. Growers need peanut cultivars adapted to West Texas growing conditions with resistance to S. minor. Methods used to evaluate resistance to S. minor in peanut included field evaluations, detached leaflet assays, and stem assays. In 2006, selected runner, higholeic spanish, and bunch-type genotypes were field tested in Stephenville, TX. Endemic inoculum was supplemented with an aggressive isolate of the most predominate Texas peanut S. minor genotype, TX1. Field disease severities were correlated with results from detached stem and leaflet assays.

Detached leaflets were inoculated with an aggressive and moderately aggressive isolate of genotype TX1 along with an aggressive isolate of TX2, another prevalent S. minor genotype. Detached stems were inoculated with the aggressive S. minor isolate TX2. For runner genotypes, a significant correlation between field ratings and detached leaflet assays occurred using the aggressive (R2 = 0.96) and moderately aggressive (R2 = 0.93) TX1 isolates when two leaflet trials (eight total replications) were combined. No correlation between field ratings and detached leaflet assays or detached stem assays occurred for spanish and bunch-types. Field ratings and detached stem assays were significantly correlated with runner genotypes only in trial I (six replications) and when trials were combined.