Zea mays inbreds resistant to different populations of Ustilago maydis (DC) Corda
--Sandoval, MC, Corcuera, VR

Ustilago maydis (Um) has produced epidemic disease in Argentina. This behaviour depended upon the maize varieties and hybrids under cultivation, as well as enviromental factors and the degree of virulence and genetic stability of the different forms of the pathogen. Rosbaco (1949) pointed out that 100% of the sweet corn grown that year in our country was attacked and Godoy (1950) reported that 14% of the maize grown in Argentina was affected by this pathogen during that season. Since then, the incidence of Um was not registered, except in maize hybrids and inbreds grown at the Institute Santa Catalina since 1989.

With the single purpose of determining the behaviour of eleven maize inbreds included in a breeding plan to obtain high quality commercial hybrids useful for different industrial procedures, pathogenicity studies were carried out, using 3 populations of Um (DC) Corda.

Eleven maize inbreds (see Table 1) were sown in pots and kept in the field. The pathogen material consisted of teliospores proceeding from spontaneous infections in Santa Catalina, Balcarce and Rio Cuarto. The teliospores were cultivated in 2% APG, and transferred to a liquid culture medium (2% CPG) and then incubated under constant agitation at 25 C until raising a concentration of 2 x 106 spores/mycelium/ml.

Once the maize plants were at the 5 leaves stage, they were inoculated using a hypodermic syringe. The inoculation was practised by puncturing the meristematic apex ( Hirschhorn, 1986) to introduce the different populations of Um. The trial consisted of 3 replicates and a tester inoculated with CPG for each inbred analyzed. A pot with 5 plants is considered a replicate.

Five days after inoculation, observations and data collection were initiated. Data obtained were analyzed through ANOVA. The determination of the degree of infection was done using the scale detailed in Table 2 (Sandoval, 1997). The statistical analysis showed no significant differences in the behaviour of the pathogen populations studied.

About the degree of infection, only the inbred 3012/8 showed severe attack (grade 4). In this case the classical tumors (galls), as well as dead plants were observed. The other inbreds studied only showed slight attack (grade1) with hypersensitivity reactions. Because of this, these inbreds can be classified as resistant or inmune (grade 0) to Um. This behaviour of the inbreds studied when inoculated with the three populations of Um suggests the existence of resistance in these maize genotypes, which will then be useful for the breeding plan under development.

Table 1. Maize inbreds inoculated with different populations of Um.
 
Inbred Description
3008/ 2/1 waxy
3008/2/6 waxy
3009 dent
3011 flint
3012 waxy
3012/8 waxy
3016/10/1 waxy
3016/5 waxy
3033 dent
3070 flint
3071/3 flint

Table 2. Scale used to determine the pathogenicity degree of Um populations based on the host plant response.
 
Population Host Reaction
Avirulent No infection - 0
Scarcely virulent Slight chlorosis on leaves and stems with or without anthocyanins. Preferentially on leaves - 1
Mid virulent Severe chlorosis with necrotic spots on leaves; negative effects on development - 2
Virulent Gall formation - 3
Very virulent Severe attack with death of plants 6 days after infection - 4


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