F. Obanor

Fusarium culmorum is a single phylogenetic species based on multilocus sequence analysis

Fusarium culmorum is a major pathogen of wheat and barley causing head blight and crown rot in cooler temperate climates of Australia, Europe, West Asia and North Africa. To better understand its evolutionary history we partially sequenced single copy nuclear genes encoding translation elongation factor 1-α (TEF), reductase (RED) and phosphate permease (PHO) in 100 F. culmorum isolates with 11 isolates of Fusarium crookwellense, Fusarium graminearum and Fusarium pseudograminearum. Phylogenetic analysis of multilocus sequence (MLS) data using Bayesian inference and maximum parsimony analysis showed that F. culmorum from wheat is a single phylogenetic species with no significant linkage disequilibrium and little or no lineage development along geographic origin. Both MLS and TEF and RED gene sequence analysis separated the four Fusarium species used and delineated three to four groups within the F. culmorum clade. But the PHO gene could not completely resolve isolates into their respective species. Fixation index and gene flow suggest significant genetic exchange between the isolates from distant geographic regions. A lack of strong lineage structure despite the geographic separation of the three collections indicates a frequently recombining species and/or widespread distribution of genotypes due to international trade, tourism and long-range dispersal of macroconidia. Moreover, the two mating type genes were present in equal proportion among the F. culmorum collection used in this study, leaving open the possibility of sexual reproduction.

Fitness of three Fusarium pathogens of wheat.

Crown rot and head blight of wheat are caused by the same Fusarium species. To better understand their biology, this study has compared 30 isolates of the three dominant species using 13 pathogenic and saprophytic fitness measures including aggressiveness for the two diseases, saprophytic growth and fecundity and deoxynivalenol (DON) production from saprophytic colonization of grain and straw. Pathogenic fitness was generally linked to DON production in infected tissue. The superior crown rot fitness of Fusarium pseudograminearum was linked to high DON production in the stem base tissue, while Fusarium culmorum and Fusarium graminearum had superior head blight fitness with high DON production in grains. Within each species, some isolates had similar aggressiveness for both diseases but differed in DON production in infected tissue to indicate that more than one mechanism controlled aggressiveness. All three species produced more DON when infecting living host tissue compared with saprophytic colonization of grain or straw, but there were significant links between these saprophytic fitness components and aggressiveness. As necrotrophic pathogens spend a part of their life cycle on dead organic matter, saprophytic fitness is an important component of their overall fitness. Any management strategy must target weaknesses in both pathogenic fitness and saprophytic fitness.

Wheat crown rot pathogens Fusarium graminearum and F. pseudograminearum lack specialization

This article reports a lack of pathogenic specialization among Australian Fusarium graminearum and F. pseudograminearum causing crown rot (CR) of wheat using analysis of variance (ANOVA), principal component and biplot analysis, Kendalls coefficient of concordance (W), and κ statistics. Overall, F. pseudograminearum was more aggressive than F. graminearum, supporting earlier delineation of the crown-infecting group as a new species. Although significant wheat line-pathogen isolate interaction in ANOVA suggested putative specialization when seedlings of 60 wheat lines were inoculated with 4 pathogen isolates or 26 wheat lines were inoculated with 10 isolates, significant W and κ showed agreement in rank order of wheat lines, indicating a lack of specialization. The first principal component representing nondifferential aggressiveness explained a large part (up to 65%) of the variation in CR severity. The differential components were small and more pronounced in seedlings than in adult plants. By maximizing variance on the first two principal components, biplots were useful for highlighting the association between isolates and wheat lines. A key finding of this work is that a range of analytical tools are needed to explore pathogenic specialization, and a statistically significant interaction in an ANOVA cannot be taken as conclusive evidence of specialization. With no highly resistant wheat cultivars, Fusarium isolates mostly differ in aggressiveness; however, specialization may appear as more resistant cultivars become widespread.

Mycotoxins produced by Fusarium spp. associated with Fusarium head blight of wheat in Western Australia

An isolated occurrence of Fusarium head blight (FHB) of wheat was detected in the south-west region of Western Australia during the 2003 harvest season. The molecular identity of 23 isolates of Fusarium spp. collected from this region during the FHB outbreak confirmed the associated pathogens to be F. graminearum, F. acuminatum or F. tricinctum. Moreover, the toxicity of their crude extracts from Czapek-Dox liquid broth and millet seed cultures to brine shrimp (Artemia franciscana) was associated with high mortality levels. The main mycotoxins detected were type B trichothecenes (deoxynivalenol and 3-acetyldeoxynivalenol), enniatins, chlamydosporol and zearalenone. This study is the first report on the mycotoxin profiles of Fusarium spp. associated with FHB of wheat in Western Australia. This study highlights the need for monitoring not just for the presence of the specific Fusarium spp. present in any affected grain but also for their potential mycotoxin and other toxic secondary metabolites.

The influence of increasing temperature and CO2 on Fusarium crown rot susceptibility of wheat genotypes at key growth stages

Despite recent reports advancing our understanding of climate change on plant diseases, uncertainty remains concerning how host and pathogen interactions are changed by increases in atmospheric carbon-dioxide (CO2) and temperature. This study has observed crown rot inoculated and non-inoculated plants in three glasshouse environments comprising ambient CO2 with ambient temperature (E1), elevated CO2 with ambient temperature (E2) and elevated CO2 with warm temperatures (E3). The proportion of crown rot infected tillers (incidence), length of stem browning (severity) and biomass of Fusarium pseudograminearum in 16 wheat genotypes was destructively assessed at node development, anthesis, soft dough and crop maturity. Mean incidence, severity and Fusarium biomass was greater in E2, and all three measurements increased at a faster rate across plant development stages; E1 showed the lowest mean incidence and severity. Incidence and severity at each development stage was dependent on the environment each genotype was grown. The influence of genotype on Fusarium biomass at each development stage however was not seen to be dependent on environment. Irrespective of genotype plants with greater severity or relative Fusarium biomass showed lower plant dry weight at crop maturity in all environments with exception to E3, where CR severity did not exert a cost to plant dry weight. These results may allude to plant maturity and temperature-dependent resistance as effective mechanisms in building resistance to crown rot. Regardless of temperature, if crown rot symptoms and Fusarium biomass are to increase at elevated CO2 there is potential for a loss in crop production capability while boosting inoculum in crop stubble.

Genetic variation in Spilocaea oleagina populations from New Zealand olive groves

Olive leaf spot caused by the fungus, Spilocaea oleagina, is the most important leaf disease of olives in many olive-growing regions worldwide with yield losses of up to 20%. The genetic structure of S. oleagina populations was investigated with universally primed-polymerase chain reaction (UP-PCR) techniques. Ninety-eight S. oleagina isolates were collected from 12 known and 4 unknown cultivars from olive groves in five New Zealand regions. UP-PCR profiles based on 159 markers were used to compute genetic distances between pairs of individuals. Low levels of gene and genotypic diversity were detected in all populations, with 76% of the loci being polymorphic and with Nei’s diversity indices ranging from 0.0234 to 0.1393. Analysis of molecular variance showed small but significant (P = 0.001) variations among regions, although most of the molecular variability (87%) was found within populations. Clustered analysis showed no evidence of grouping according to geographic origin of the isolates. The low level of genetic diversity found within and among populations indicates that reproduction for this fungus is predominantly by asexual means and that any effective control strategies are likely to be useful in all or most New Zealand olive groves.

Sources of Botrytis cinerea inoculum for flower infection in blackcurrants in New Zealand

Botrytis cinerea is a major disease of blackcurrants causing premature flower and fruit drop in New Zealand. Because of the risk of fungicide residue from conventional methods, there is a need for the development of non-chemical control strategies as part of a sustainable disease control programme. Potential sources of Botrytis cinerea inoculum for infection of blackcurrant flowers were sampled in five blackcurrant properties in the South Island of New Zealand during dormancy and spring of 2006 and 2007. Tissues investigated were necrotic debris immediately beneath the plant, necrotic growing tips of 1-year-old canes, dead cane of previous seasons growth still attached to the base of the plant, canes damaged by harvesting or pruning, and canes damaged by currant clearwing (Synanthedon tipuliformis). The mean number of each tissue type within a plant varied considerably, both within a property and between different properties. Necrotic debris immediately beneath the plant and necrotic cane tips were shown to be the main sources of inoculum. Knowledge of the main sources of inoculum is an important first step towards the development of new, non-chemical control strategies, which are discussed in this context.