Morph-agronomic characterization of watermelon accessions with resistance to Fusarium Wilt.

Fifty-five watermelon accessions were inoculated in pots using Fusarium oxysporum f. sp. niveum chlamydospores in a completely randomized design with five replications. Among the 42 accessions that were resistant, twelve accessions with a mean score ≤1 were selected for morpho-agronomic characterization. 'Sugar Baby' and 'Charleston Gray' were used as controls. Field research was conducted over two production cycles in a randomized block design with three replications and five plants per plot. For the characterization 13 quantitative agronomic traits were used. The dissimilarity measures were obtained by the sum of the matrices of standardized mean Euclidean distance. Coefficients of variation ranged from 5.33% (number of days until 50% of plants with at least one female flower) and 17.20% (mass of characterized fruit). In the second cycle, a reduction in days was observed for the flowering. For soluble solids content, the accession BGH-UNIVASF 40 was statistically equal to the commercial cultivars in the second cycle. Accessions were grouped similarly using two grouping methods, Tocher and the farthest neighbor method. The most promising accessions were BGH-UNIVASF 40, 169, 177 and 210 for use in future hybridizations. BGH-UNIVASF 76, 128 and 185 (Citrullus lanatus var. citroides) can be used as resistant Fusarium wilt rootstocks.

Colletotrichum lindemuthianum exhibits different patterns of nuclear division at different stages in its vegetative life cycle.

Live-cell imaging with fluorescent protein labeling is providing major new insights into nuclear dynamics in filamentous fungi. With this approach we provide a detailed report of nuclear organization and behavior during mitosis in the bean pathogen Colletotrichum lindemuthianum. Nuclear division and nuclear migration were analyzed in ungerminated conidia, conidial germlings and the mature colony. Ungerminated conidia were uninucleate and completion of mitosis was found not to be essential for germ tube formation, conidial anastomosis tube (CAT) formation or fusion. Nuclei in fused conidial germlings exhibited asynchronous mitoses, and nuclear migration through fused CATs occurred after the nuclei had divided. Different patterns of nuclear division were found in vegetative hyphae of the mature colony. Synchronous, parasynchronous and asynchronous patterns of mitosis were observed in apical hyphal compartments at the colony border, while only synchronous and asynchronous mitoses occurred in subapical hyphal compartments. These findings have revealed unexpected diversity in the patterns of mitosis in different cells of C. lindemuthianum.

Heterokaryon incompatibility is suppressed following conidial anastomosis tube fusion in a fungal plant pathogen.

It has been hypothesized that horizontal gene/chromosome transfer and parasexual recombination following hyphal fusion between different strains may contribute to the emergence of wide genetic variability in plant pathogenic and other fungi. However, the significance of vegetative (heterokaryon) incompatibility responses, which commonly result in cell death, in preventing these processes is not known. In this study, we have assessed this issue following different types of hyphal fusion during colony initiation and in the mature colony. We used vegetatively compatible and incompatible strains of the common bean pathogen Colletotrichum lindemuthianum in which nuclei were labelled with either a green or red fluorescent protein in order to microscopically monitor the fates of nuclei and heterokaryotic cells following hyphal fusion. As opposed to fusion of hyphae in mature colonies that resulted in cell death within 3 h, fusions by conidial anastomosis tubes (CAT) between two incompatible strains during colony initiation did not induce the vegetative incompatibility response. Instead, fused conidia and germlings survived and formed heterokaryotic colonies that in turn produced uninucleate conidia that germinated to form colonies with phenotypic features different to those of either parental strain. Our results demonstrate that the vegetative incompatibility response is suppressed during colony initiation in C. lindemuthianum. Thus, CAT fusion may allow asexual fungi to increase their genetic diversity, and to acquire new pathogenic traits.

Live-cell imaging of conidial fusion in the bean pathogen, Colletotrichum lindemuthianum.

Fusion of conidia and conidial germlings by means of conidial anastomosis tubes (CATs) is a common phenomenon in filamentous fungi, including many plant pathogens. It has a number of different roles, and has been speculated to facilitate parasexual recombination and horizontal gene transfer between species. The bean pathogen Colletotrichum lindemuthianum naturally undergoes CAT fusion on the host surface and within asexual fruiting bodies in anthracnose lesions on its host. It has not been previously possible to analyze the whole process of CAT fusion in this or any other pathogen using live-cell imaging techniques. Here we report the development of a robust protocol for doing this with C. lindemuthianum in vitro. The percentage of conidial germination and CAT fusion was found to be dependent on culture age, media and the fungal strain used. Increased CAT fusion was correlated with reduced germ tube formation. We show time-lapse imaging of the whole process of CAT fusion in C. lindemuthianum for the first time and monitored nuclear migration through fused CATs using nuclei labelled with GFP. CAT fusion in this pathogen was found to exhibit significant differences to that in the model system Neurospora crassa. In contrast to N. crassa, CAT fusion in C. lindemuthianum is inhibited by nutrients (it only occurs in water) and the process takes considerably longer.