Fungus gnats in forestry nurseries and their possible role as vectors of Fusarium circinatum

Abstract

There are many examples of associations between insects and fungi. Where the fungi involved are pathogens, such associations may be of economic importance. Insects of no economic concern alone can also become important pests because of their association with fungal pathogens. Insects may assist in the spread of pathogens by carrying them on or in their bodies. Insects may also predispose plants to infection by creating wounds during feeding, oviposition or other behavioural activities. Knowledge of associations between insects and fungal pathogens often form a crucial component in the management strategy of these pathogens. The pitch canker fungus, Fusarium circinatum, causes severe disease symptoms on mature pines in the USA. Various insects have been implicated as vectors of this disease. In South Africa, F. circinatum is reported only to cause disease on pine seedlings, where it results in severe losses in nurseries. Various insects are present in the nursery that could possibly be associated with the spread of the fungus or the infection of its hosts. Amongst these insects, fungus gnats are the prime suspects due to their history of association with fungal pathogens in other nurseries. The presence of fungus gnats in South African pine nurseries and their possible association with F. circinatum and other pathogens has never been investigated critically. The objective of this study was to expand the base of knowledge of fungus gnats in South African pine nurseries, and to consider their possible association with F. circinatum and their population structure within and between nurseries. The literature review provides a summary of fungus gnats in the nursery environment. This includes their description, biology and association with fungal pathogens. Information from these studies is used to evaluate the possible association between fungus gnats and F. circinatum in South African pine nurseries. In nurseries around the world where fungus gnats are considered pests, various control options have been used, and these are further discussed. The first research aim of this study was to determine whether fungus gnats are present in the major pine nurseries of South Africa. Thus, in Chapter 2, surveys were undertaken in four of the major pine nurseries. All fungus gnats collected were identified to species level. Other diptera collected were identified to family level. Furthermore, all diptera collected were isolated on general and selective growing medium to examine for the presence of F. circinatum. Results from Chapter 2 showed that only one species of fungus gnats was present in the nurseries and it was present in all four of the nurseries surveyed. This raised interesting questions regarding the phylogeographic structure of these populations and the diversity within and between populations. These questions are addressed in Chapter 3 using analysis of mitochondrial COI sequence data from fungus gnats collected in the four nurseries. Of particular importance was the interpretation of these results as it pertains to the movement of fungus gnats between populations, together with their associated fungi. Using general and specific growing medium to isolate fungal pathogens from insects is not necessarily an accurate method. Pathogens may be overgrown by faster growing fungi before they are noticed, especially if they are present only in small amounts. Chapter 4 examined the use of DNA-based methods as a tool to detect fungal pathogens on fungus gnats. Fungus gnats were collected from the same four nurseries as in Chapter 2. Species-specific primers for F. circinatum and Botrytis cinerea were used to detect these fungi. Dilution series were done to examine the sensitivity of the primers. General primers were used to detect other fungi. This dissertation includes some of the first studies ever undertaken on fungus gnats in South African pine nurseries. Their association with the very virulent pitch canker fungus is also considered in some detail. It is my hope that these studies will form a foundation for future research on fungus gnats in South Africa. Copyright