Maize Genetics Cooperation Newsletter 80. 2006.

HONOLULU, HAWAII

University of Hawaii

DAEJEON, SOUTH KOREA

Chungnam National University

Segregation of resistance to southern corn rust in set M RIL population

--Ji, HC; Brewbaker, JL

Southern corn rust (Puccinia polysora Underwood) becomes severe in the wet, somewhat cooler winter months (mean temp. 72�F) of Hawaii. Previous evaluations of several hundred tropical and temperate inbreds revealed three distinct levels of general resistance (not racially-specific)—highly susceptible, highly resistant and intermediate. Highly susceptible lines (e.g., North American sweet corns) have lesions on all leaves and often fail to set seed. Some inbreds (normally tropical in origin) were highly resistant, showing very few lesions and only on the upper surface of lower leaves. An intermediate level of tolerance was also common among tropical inbreds (summarized by Brewbaker, Kim and Logrono in Hawaii Res. Series 62, 1989). All tested monogenes for rust tolerance (e.g., Rpp9) were ineffective in Hawaii.

Set M of recombinant inbreds (RILs) was created in Hawaii from a hybrid of two supersweet inbreds, Iowa�s Ia453sh2 and Hawaii�s Hi38c1bt, each a significant parent for commercial hybrids. The temperate inbred was highly susceptible while the tropical inbred showed an intermediate level of tolerance that minimized yield losses to sweet corn producers in Hawaii. The parents and 55 RILs (six generations selfed) were planted in a three-rep RCB at Waimanalo Research Station (sea level) in the winter of 2003. The RILs from the very poorly adapted Iowa inbred were often very poor in vigor, intolerant of the low-light winter conditions and the high incidence of fusarium rots in this season.

Rust was scored on an empirical scale of 1 (highly tolerant) to 9 (highly susceptible), rating the entire plant. The parents averaged 3.1�1.58 (Hi38c1bt) and 5.3�2.09 (Ia453sh2). The 55 RILs ranged from 2.0 to 7.0 (Fig. 1), with an average of 4.45 and a dis

Hi38c1

Ia453 sh2

Figure 1. Frequency distribution of resistance to southern corn rust on RIL set M.

tribution skewed toward susceptibility. We reported similar data (So, Ji and Brewbaker, MNL 77:32-33) for a generation mean analysis of a cross of Hi38-71, sister inbred of Hi38c1, with a susceptible field corn inbred, G24. Their rust scores (3.2 for Hi38-71, 6.9 for G24) were generally similar to those seen here. In that study the heterotic vigor of segregating progenies tended to bias the rust-readings toward resistance (F1 average was 2.9, F2 average was 4.2), and estimated gene number was 2.8. In contrast, the generally weak inbreds in the current study tended to be scored with bias toward susceptibility. It is possible to view the present data as two groups of inbreds, one tolerant (with disease scores lower than or equal to 4.4) and the other susceptible (with disease scores higher than 4.4). The distribution of RILs fitted a single-gene model tested by the normal frequency curve method (Moon, Maydica 44:301, 1999), but a digenic model appears more likely.

Inbreds such as parent Hi38c1 showed many lesions on lower leaves, while leaves above the ear were often quite clean of rust. This tolerance has been described as �mature-plant resistance� or �late-rusting�. Careful digital estimates of lesion areas on a major leaf (e.g., subtending the ear) would probably distinguish genotypes more clearly, and the evaluation of vigorous testcrosses might best lead to their convincing distinction. In practice the tolerance of our �tolerant� RILs is adequate for production under severe epiphytotics in Hawaii.

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