Influence of climatic factors on dry flower, grey and green mould diseases of macadamia flowers in Australia.

AIMS: Flower blights (grey mould, green mould and dry flower) are important diseases of macadamia. Lack of information on pathogen biology and disease epidemiology in macadamia has hampered control options. Effects of climatic variables including temperature, relative humidity (RH) and vapour pressure deficit (VPD) on the abundance, germination and growth of conidia of four fungal pathogens that cause various flower blights in macadamia were studied. METHODS AND RESULTS: Mycelial growth, sporulation, conidial germination and germ tube growth for five isolates each of Botrytis cinerea, Cladosporium cladosporioides, Pestalotiopsis macadamiae and Neopestalotiopsis macadamiae, at eight temperatures, seven RH and the corresponding VPD regimes were determined in vitro. The optimal climatic range of each of the four pathogens was validated during macadamia flowering periods in the 2019 and 2020 seasons by conidia detected and quantified using quantitative PCR. Several growth models were fitted to the data with high significance; predicted optima from these models ranged from 0.9 to 1.1 kPa VPD for P. macadamiae and N. macadamiae and <0.6 kPa VPD for B. cinerea and C. cladosporioides. CONCLUSIONS: This study showed that VPD, as a determinant of the fecundity and growth of the four fungal pathogens, was predictive of flower blight incidence in macadamia. The importance of temperature, RH and, thus, VPD for defining the conditions for infection and flower blight epidemics was established. SIGNIFICANCE AND IMPACT OF THE STUDY: This information provides a firm basis for the development of prediction tools for flower blights in macadamia.

Sources, Detection, and Inoculum Quantification of Flower Blight Pathogens in Macadamia.

Dry flower disease caused by Pestalotiopsis/Neopestalotiopsis spp., green mold caused by Cladosporium spp., and gray mold caused by Botrytis spp., collectively known as flower blight cause significant yield losses in macadamia. Potential sources of inoculum of the various pathogens in macadamia tree canopy were examined using pathogenicity tests and a multiplex quantitative PCR (qPCR) assay developed in this study. The qPCR assay detected and quantified the relative abundance of the inoculum of flower blight pathogens. The assay revealed that remnant racemes contributed a high amount of inoculum of all the three groups of flower blight pathogens, while the yellow halo leaf spot contributed only Pestalotiopsis/Neopestalotiopsis species. The amount of conidia per gram of remnant racemes ranged from 7 × 103 to 2 × 104 for dry flower disease, 3 × 103 to 1 × 104 for green mold, and 5 to 8 × 103 for gray mold pathogens. Conidia of Pestalotiopsis/Neopestalotiopsis species quantified from leaf spots varied from 1 × 102 to 1 × 103 per cm2. Pathogenicity tests performed on developing racemes under field conditions, using conidial suspensions from both sources of inoculum (remnant racemes and yellow halo leaf spot), resulted in severe flower bight symptoms. Disease severity was not significantly different (P > 0.05) when remnant racemes were incubated directly with the developing racemes compared with inoculation with conidial suspension from the material. This suggests that racemes from preceding seasons that remain in the tree canopy carryover inoculum between seasons and should be removed as a control option for flower blights in macadamia orchards.

Neopestalotiopsis Species Associated with Flower Diseases of Macadamia integrifolia in Australia.

Macadamia (Macadamia integrifolia) is native to eastern Australia and produces an edible nut that is extensively cultivated in commercial orchards in several countries. Little is known about the diversity of fungi associated with diseases of macadamia inflorescences. A survey of fungi associated with the dry flower disease of macadamia detected several isolates of Neopestalotiopsis (Pestalotiopsidaceae, Sordariomycetes). Five new species of Neopestalotiopsis were identified based on molecular phylogenetic analyses of concatenated gene sequences of the internal transcribed spacer (ITS), ß-tubulin (TUB), and the translation elongation factor 1-alpha (TEF1α). The new species are named Neopestalotiopsis drenthii, N. maddoxii, N. olumideae, N. vheenae, and N. zakeelii, and are described by molecular, morphological, and cultural characteristics. The ecology of the isolates and their pathogenic, saprophytic, or commensal ability were not determined.

Novel Botrytis and Cladosporium Species Associated with Flower Diseases of Macadamia in Australia.

Macadamia (Macadamia integrifolia) is endemic to eastern Australia and produces an edible nut that is widely cultivated in commercial orchards globally. A survey of fungi associated with the grey and green mold symptoms of macadamia flowers found mostly species of Botrytis (Sclerotiniaceae, Leotiomycetes) and Cladosporium (Cladosporiaceae, Dothideomycetes). These isolates included B. cinerea, C. cladosporioides, and unidentified isolates. Amongst the unidentified isolates, one novel species of Botrytis and three novel species of Cladosporium were delimited and characterized by molecular phylogenetic analyses. The new species are Botrytis macadamiae, Cladosporium devikae, C. macadamiae, and C. proteacearum.

Assessment of management practices, awareness on safe use of pesticides and perception on integrated management of pests and diseases of chilli and tomato grown by small-scale farmers in selected districts of Sri Lanka.

BACKGROUND: In this study, we surveyed through a structured and pilot-tested questionnaire, the practices of pesticide usage by small-scale (< 1.25 ha) chilli and tomato farmers from four districts of Sri Lanka and their perceptions on the effectiveness of pesticides and willingness to adopt integrated pest management (IPM). RESULTS: We found significant (P < 0.05) variation among districts in farmer responses to 37 out of 59 survey questions. A majority of farmers were dependent on pesticides with only a minority practicing IPM. A majority perceived that their current pesticide usage was increasing, but was not excessive. A majority were aware of the negative impacts of pesticides on human health and environment. Farmer perception on the effectiveness of pesticides increased with age, but was not influenced by education level. Farmers having a higher opinion of pesticides made a greater number of applications during a cropping cycle. They also maintained a longer preharvest interval and did not use pesticide mixtures. A majority expressed willingness to initiate IPM, but identified lack of knowledge and technical knowhow on specific IPM practices for their crops as barriers to adoption and requested external support. Farmer willingness to adopt IPM is higher among older, more educated farmers and among full-time farmers who are currently totally-dependent on pesticides and whose major income source was farming. CONCLUSION: We conclude that a significant extension effort in terms of farmer education on IPM and external assistance to develop the technological knowhow, which is tailor-made to specific districts, is needed to facilitate adoption of IPM among these farmers.

Recent Advances in Molecular Diagnostics of Fungal Plant Pathogens: A Mini Review.

Phytopathogenic fungal species can cause enormous losses in quantity and quality of crop yields and this is a major economic issue in the global agricultural sector. Precise and rapid detection and identification of plant infecting fungi are essential to facilitate effective management of disease. DNA-based methods have become popular methods for accurate plant disease diagnostics. Recent developments in standard and variant polymerase chain reaction (PCR) assays including nested, multiplex, quantitative, bio and magnetic-capture hybridization PCR techniques, post and isothermal amplification methods, DNA and RNA based probe development, and next-generation sequencing provide novel tools in molecular diagnostics in fungal detection and differentiation fields. These molecular based detection techniques are effective in detecting symptomatic and asymptomatic diseases of both culturable and unculturable fungal pathogens in sole and co-infections. Even though the molecular diagnostic approaches have expanded substantially in the recent past, there is a long way to go in the development and application of molecular diagnostics in plant diseases. Molecular techniques used in plant disease diagnostics need to be more reliable, faster, and easier than conventional methods. Now the challenges are with scientists to develop practical techniques to be used for molecular diagnostics of plant diseases. Recent advancement in the improvement and application of molecular methods for diagnosing the widespread and emerging plant pathogenic fungi are discussed in this review.