Ustilago maydis (Um) is a Basidiomycete that promotes the development of galls in Zea, the relation with the host being necessary to fulfill its life cycle. Damage produced in maize plants by the presence of corn stunt are: chlorosis, seedling death, tassel seed and tumors in leaves, stems, ears and tassel. At first, it was thought that Um attacked only Z. mays and Z. mexicana, but later it was verified that Um also attacks Z. perennis, Z. diploperennis, Z. parviglumis, Z. luxurians and their hybrids with the grown species.
Until 1964, corn stunt did not have any incidence at the IFSC. But in that year, a Z. perennis clone from Jalisco (Mexico) was introduced and later on Z. mexicana, Z. parviglumis, Z. luxurians and Z. diploperennis were also grown and hybridized to Z. mays. As the hybrids are grown at the field as well as in the greenhouse, vegetative plants are available during all the year. The pathogen multiplies on these plants with a corresponding increase in the number of spores disseminated by air and in the soil. This fact is the main cause by which 1 to 10% of all Zea species and hybrids are attacked, depending on environmental conditions favouring pathogen development.
In this paper the results of analyzing the response of Z. mays, Z. perennis and Z. diploperennis seedlings when they are inoculated with 6 populations of Um are presented. This was done with the purpose of determining resistance of the species and/or inbreds to Um.
The host materials used were the population cv "Colorado Klein", the inbreds SC66, B73, E624A688 of Z. mays as well as clones of Z. perennis and Z. diploperennis. 1296 plants were inoculated during two years running (1997-98) with different strains of Um isolated from the province of Buenos Aires (Sta. Catalina, Balcarce, Necochea and 25 de Mayo), the province of Entre Ríos (Paraná) and the province of Córdoba (Río Cuarto). These strains were cultivated in a liquid medium of 2% C.P.G under shaking during 8 days running at 25 C ± 2. The pathogen was inoculated by puncturing the base of the seedlings with an hypodermic syringe and the sporidial suspension was then forced up into the leaf whorl. In many previous works, this method was really successful in producing disease galls in seedlings. The trial involved three replications and a tester (non-treated plants). The plants were evaluated using a reaction scale to determine the mean percentage of infection with Um (Table 1). First symptoms in seedlings were observed 3 to 4 days after inoculations and gall development occurred 7 to 8 days after the treatment.
The behaviour of the host when inoculated with 6 populations of Um is analyzed in Figures 1-6. The hosts that reacted forming galls (grade 4) were cv Colorado Klein: Necochea (8.34%) and Balcarce (2.78%); B73: Río Cuarto (14.15%), 25 de Mayo (11.11%), Sta. Catalina (5.84%) and Balcarce (1.04%); E642A688: 25 de Mayo (8.33%) and Sta. Catalina (3.34%); SC66: Rio Cuarto (4.55%); Z. perennis: Sta. Catalina (1.67%) and Z. diploperennis: 25 de Mayo (13.89%), Paraná (2.78) and Sta. Catalina (1.67%).
From this analysis, it can be concluded that the wild and grown species of the genus Zea reacted in different ways (tolerant and/or resistant to moderately susceptible) depending on the geographic origin of Um populations.
These results might be considered when selecting germplasm to obtain new forage plants from interspecific hybrids of the genus Zea.
Table 1. Reaction scale in hosts.
| Behaviour | Host Reaction |
| 0= Inmune | No reaction. |
| 1= Resistant | Partial chlorosis. |
| 2= Medium resistant | Accent chlorosis and/or presence of stripe or anthocyanin stain. |
| 3= Medium susceptibly | Necrosis and disminution of growth in plant. |
| 4= Susceptibly | Formation of tumors (galls) |
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