During the growing seasons 1996/97 and 1997/98, seventy-eight maize inbreds were evaluated for their response to the attack of the fungal pathogens responsible for corn smut and maize rust. These inbreds are being developed and tested to obtain commercial hybrids with high protein or starch content and quality. In the period mentioned above, data from all the plants belonging to different pedigrees were taken to determine percentage of attack and placement of the tumors produced by Ustilago maydis (DC) Corda (Figure 1). Only 7.95% were attacked by this pathogen. This figure is lower than the average registered at the Institute (13.33%). Considering only the diseased plants, 53% of them showed tumors in leaves and the stems, 36% showed ear tumors and only 10% denoted tumors in ears and anthers as well as the presence of female flowers in the tassel (tassel-seed) and the development of structures similar to anthers in the apex of the ears. This phenomenon could be attributed to the production of hormones or metabolites by the pathogen which would promote the hormonal alteration of the host.

78% of all the genotypes studied showed rust attack in grade 2 and 3 (Figure 2). Chlorotic spotting phenotype was observed in 63% of these materials (Figure 3). 51% of them also showed the classic pustules and only 13% of the pedigrees revealed chlorotic spotting phenotype as unique sympton.

Table 1 shows the results obtained for corn stunt and maize rust attack in 14 inbreds selected from the whole analyzed by their upper agronomic traits. These inbreds were also measured during both growing seasons. An increase of Puccinia sorghi attack was registered during the second season. This could be due to the favourable temperature and humidity conditions which helped teleospore germination. Three of these inbreds were irradiated with 150 Gy at a Gamma-cell (Co60). The irradiated inbred 3089 showed scarce attack by P. sorghi if compared to the original genotype. Almost no changes were appreciated in the inbred 3074 when irradiated and the degree of attack became higher in the irradiated 3078/5.

Recessive mutations can be obtained using ionizing radiation. Dominant mutants are rarely developed by this method. The interaction between the original background of the inbreds and the radiation dose and type are responsible for the different behaviour of the plants subjected to the same treatment.

Not only does specific resistance to P. sorghi exist, but there is also general resistance controlled by multiple genes (Gingera et al., 1995), which appears both in adult plants and seedlings as small and isolated pustules that constitute the so-called "chlorotic spotting phenotype". Many times these symptons were confused with a degree of susceptibility to rust, but they really constitute a resistance form. Ears from plants denoting this phenotype were already selected to continue improving resistance to maize rust in the materials employed in this breeding

Table 1. Maize rust and corn stunt incidence on 14 inbreds analyzed during 1996/97 and 1997/98.
 

INBRED	PUCCINIA SORGHI	USTILAGO MAYDIS
3003 A	45.0	0.0
B	56.5	0.0
3002 A	43.7	12.5
B	100.0	0.0
3012/7 A	38.4	7.7
B	50.0	0.0
3016/7 A	50.0	0.0
B	25.0	0.0
3016/1 A	72.2	0.0
B	25.0	0.0
3022 A	60.0	0.0
B	62.5	0.0
3024 A	64.0	0.0
B	55.9	0.0
3078/1 A	50.0	0.0
B	86.5	0.0
3078/2 A	57.8	0.0
B	50.0	0.0
3072 A	56.6	0.0
B	83.9	6.5
3102 A	62.5	6.2
B	80.9	0.0
3074 A	94.7	0.0
B	68.7	0.0
C	61.5	2.6
3078/5 A	61.5	2.5
B	44.5	0.0
C	84.2	10.5
3089 A	64.0	0.0
B	94.2	0.0
C	27.8	0.0

A: data belonging to growing season 1996/97, B: data belonging to growing season 1997/98, C: data belonging to irradiated plants measured during 1997/98.

Figure 1. Tumors of corn smut.

Figure 2. Maize rust, classic symptom.

Figure 3. Chlorotic-spotting phenotype.

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