Behaviour of maize inbreds and single-crosses to spontaneous infections of Puccinia sorghi Schw.

--Sandoval, MC, Corcuera, VR, Bernatené, EA, Naranjo, CA

During 1990, at the Instituto Fitotécnico de Santa Catalina, a maize quality breeding plan aimed at obtaining high nutritional and high added value commercial maize hybrids was initiated. Studying the behaviour of the inbreds and single crosses in relation to the most important diseases constitutes a main objective of the program. During the growing season of 2002/03, 32 inbreds and 60 hybrids sown in a three-replicate randomized block design field trial at Llavallol at a density of 71,500 plantas/ha were studied for their behaviour to spontaneous infection of Puccinia sorghi Schw. (Figure 1), the etiological agent responsible for “common leaf rust.” The maize genotypes studied may be classified by the nature of their endosperm as: a) waxy, b) high quality protein and c) waxy and high quality protein (double recessive). In maize regions with temperate winters like Argentina, the whole cycle of the disease may be verified. In our country, severity levels for common rust are near 40%.

The following parameters were analyzed: 1) Type or degree of infection based on a modified Cobb’s scale

severity% Type of Infection Behaviour to P. sorghi
less than 5 I Very resistant
6 to 10 II Resistant
11 to 25 III Mid-resistant
26 to 40 IV Mid-susceptible
41 to 65 V Susceptible
66 to 100 VI Very susceptible

and 2) percentage severity index (PSI) according to Wheeler (1969) and calculated as follows: sum of numerical ratings x 100/No of plants scored × Maximum score on scale.

The PSI (%) varied from 0 to 55 within the inbreds analyzed. Nine percent of them (3022c, 3074c and 3072) were very resistant, 28% resistant, 31% moderately resistant, 16% moderately susceptible (3096c, 3141, 3002c, 3074a and 3142), 3% susceptible (3014) and 13% immune (3139 II and 3024). The PSI (%) ranged from 0 to 32 in the hybrids. Eighty-three percent of them were very resistant, 3% resistant (precommercial hybrids 3222** and 3222***), 2% moderately resistant (precommercial hybrid 3181**) and 12% immune (precommercial hybrids 3231, 3191*, 3165, 3164, 3165* and 3215).

Pairwise correlations between PSI and potential yield of the single-crosses evaluated were performed. A negative correlation value (r: -0.265) was found, although it is not statistically significant. The fact that in some genotypes PSI was zero, does not let us assert categorically the existence of immunity to illness, as this can also be due to the presence of some other mechanism of evasion that was not considered; this would be worth evaluating in depth in our next studies. All the same, the behaviour of most of the genotypes under evaluation may be detected according to PSI values obtained and their behaviour to the spontaneous presence of P. sorghi.

Figure 1. Detail of uredosporic pustules of P. sorghi, developed (during the reproductive stage of cultivation) on the leaf of a waxy maize hybrid.