Infection of the cones and seeds of Welwitschia mirabilis by Aspergillus niger var. phoenicis in the Namib-Naukluft Park

Abstract

Welwitschia mirabilis Hook. fil. is a unique and rare dioecious desert gymnosperm endemic to the Namib Desert. The female plants bear 90–100 megasporophylls, of which 50–60% may be fertile, but up to 80% of those fertile seeds may be infected by Aspergillus niger var. phoenicis. This contamination results in seed and seedling death, potentially negatively affecting recruitment of plants into the population. The pattern of infection of the cones and seeds in the field was studied over a period of eight months at the Hope Mine in the Namib-Naukluft Park. Infection of the cones was found to peak coincident with the appearance of the pollination drops, and with high temperatures, winds and significant rainfall. W. mirabilis cones were microscopically examined and spores of A. niger var. phoenicis were found embedded in the dehydrating pollination drop at the tip of the micropyle, suggesting that this was one of the routes by which the seeds were infected. The heteropteran, Probergrothius sexpunctatis, was also implicated in the infection of the seeds, since it was found to be carrying spores of A. niger var. phoenicis and was observed feeding on mature seeds and immature cones. While most post-harvest treatments have proved ineffective in eradicating A. niger var. phoenicis from the seeds, promising results are presently reported using a fungicide containing tebuconazole as the active ingredient.