1Russell Agricultural Research Center, USDA/ARS, PO Box 6477, Athens, GA 30604, USA; 2Department of Botany, University of Georgia, Athens, GA 30602, USA

Background and objectives
Fusarium moniliforme infections of maize are of agricultural concern because the fungus produces mycotoxins which are harmful to animal and human health. Despite a century of research on the biology and chemistry of this fungus, methods do not exist for effectively controlling maize infections by F. moniliforme. Investigations focused on the plant side of this host-pathogen interaction could aid in developing control measures for fungal infection. The objective of this study was to analyse the host-fungal interaction for specificity of colonization relative to maize tissue, maize genotype and F. moniliforme strain.

Materials and methods
Plants of three maize genotypes were collected at three field sites located in piedmont and coastal regions of Georgia, USA. Segments of 1 cm were dissected from vegetative and reproductive tissues. Vegetative tissues included roots, stems and leaves. Reproductive tissue included developing cobs, male florets and anthers. Dissected, surface-sterilized plant tissues were inoculated with F. moniliforme conidia collected from cultures of two strains, RRC PAT and MRC 826, each transformed with the genes coding for B-glucuronidase (GUS) and hygromycin resistance [1]. A 5 l water suspension containing 106 conidia ml-1 was placed in the centre of each plant tissue segment and incubated in the dark at 20C. Mycelial growth was monitored daily for 7 days following inoculation.

Results and conclusions
Colonization of vegetative tissues. For the host-fungal interaction of PAT-sweet maize, mycelial proliferation was not visible on intact vegetative tissue surfaces for at least 5 days. Mycelia were evident on the cut edges of leaves within 3 days, but not on stems or roots. Even after 7 days, only a rare root or stem had visible mycelia on the cut edges and never on the surface. Scant mycelia were apparent on intact leaf surfaces after about 5 days.

Colonization of reproductive tissues. Of all reproductive and vegetative tissues examined, immature cobs were colonized most rapidly and abundantly. Visible mycelia were evident on immature cobs by about 2 days after inoculation, but the rate and extent of mycelial proliferation decreased with increasing cob maturity. Mycelia were visible on anthers within at least 4 days, but only at the tip and base of intact male florets by 7 days.

Regardless of maize genotype or tissue type, colonization was more rapid and more abundant for the fungal strain PAT than for 826. In addition, all tissue types of sweet maize were colonized more rapidly and abundantly on sweet corn compared to field corn.

Examination of three different maize genotypes inoculated with two different F. moniliforme strains have demonstrated that (i) reproductive tissue is more susceptible to colonization than vegetative tissue; (ii) sweet maize is more susceptible than field maize; (iii) immature cobs are more susceptible than mature cobs; and (iv) fungal strains differ in their capacity to colonize the same tissue. Thus the colonization process is affected by maize genotype, tissue type and F. moniliforme strains. Consequently the maize plant may possess characters to incorporate into strategies designed to control the levels of F. moniliforme mycotoxins in our food and feed supply.

1. Yates I et al., 1998. Mycological Research, in press.