Working with Mycorrhizas in Forestry and Agriculture

Crop Production/Industries,

Work and progress of the Agricultural Experiment Station for the year ending June 30, 1914

Cover title

Developing and communicating strategies for controlling virus diseases in vegetable cucurbit crops

Virus diseases cause serious yield and quality losses in field grown cucurbit crops worldwide. In Australia, the main viruses of cucurbits are Papaya ringspot virus (PRSV), Squash mosaic virus (SqMV), Watermelon mosaic virus (WMV) and Zucchini yellow mosaic virus (ZYMV). Plants infected early have severely distorted fruit. High infection incidences, of ZYMV and PRSV in crops cause losses of marketable fruit of up to 100% and infected crops are often abandoned. Two new alternative hosts of ZYMV were identified, the native cucurbit Cucumis maderaspatanus and wild legume Rhyncosia minima. No new alternative hosts of PRSV, SqMV or WMV were found in Western Australia or Queensland. Seed transmission of ZYMV (0.7%) was found in seedlings grown from ZYMV-infected fruit of zucchini but not of pumpkin. None was detected with PRSV or SqMV in zucchini or pumpkin seedlings, respectively. ZYMV spread to pumpkins by aphids was greater downwind than upwind of a virus source. Delaying sowing by 2 weeks decreased ZYMV spread. Millet non-host barriers between pumpkin plantings slowed ZYMV infection. Host resistance gene (zym) in cucumber cultivars was effective against ZYMV. Pumpkin cultivars with resistance gene (Zym) became infected under high virus pressure but leaf symptoms were milder and infected plants higher yielding with more market-acceptable fruit than those without Zym. Most zucchini cultivars with Zym developed severe leaf and fruit symptoms. ZYMV, PRSV, WMV and SqMV spread readily from infected to healthy cucurbit plants by direct leaf contact. ZYMV survives and remains infective on diverse surfaces for up to 6 hours but can be inactivated by some disinfectants. Phylogenetic analysis indicates at least three separate introductions of ZYMV into Australia, with new introductions rarely occurring. ZYMV isolates clustered into three groups according to collection location i) Kununurra, ii) Northern Territory and iii) Carnarvon, Qld and Vic. A multiplex Real-Time PCR was developed which distinguished between the three groups of Australian isolates. Integrated disease management (IDM) strategies for virus diseases of vegetable cucurbit crops grown in the field were improved incorporating the new information gathered. These strategies are aimed at causing using minimal extra expense, labour demands and disruption to normal practices

Plant viruses and viroids in Japan

An increasing number of plant viruses and viroids have been reported from all over the world due largely to metavirogenomics approaches with technological innovation. Herein, the official changes of virus taxonomy, including the establishment of megataxonomy and amendments of the codes of virus classification and nomenclature, recently made by the International Committee on Taxonomy of Viruses were summarized. The continued efforts of the plant virology community of Japan to index all plant viruses and viroids occurring in Japan, which represent 407 viruses, including 303 virus species and 104 unclassified viruses, and 25 viroids, including 20 species and 5 unclassified viroids, as of October 2021, were also introduced. These viruses and viroids are collectively classified into 81 genera within 26 families of 3 kingdoms (Shotokuvirae, Orthornavirae, Pararnavirae) across 2 realms (Monodnaviria and Riboviria). This review also overviewed how Japan’s plant virus/viroid studies have contributed to advance virus/viroid taxonomy

Investigations on Aspergillus fumigatus double-stranded RNAs and their effects on the fungus

The aim of this research was to assess the incidence of dsRNA mycoviruses in the opportunistic human pathogenic fungus Aspergillus fumigatus, where previously no dsRNA viruses had been reported and to investigate the effects of any dsRNAs on the growth and pathogenicity of the fungus. Thus far 366 isolates (clinical and environmental) have been screened, 24 of which posses dsRNA elements. Successful efforts were made to completely characterise the two dsRNA segments of the isolate 88, partitivirus to obtain novel sequence information. Fungal viruses or mycoviruses are widespread and they usually infect their hosts persistently without any detectable phenotypic effects. They have been however linked with both hypovirulence and hypervirulence but are normally cryptic. To obtain information on the effect of the dsRNAs on their respective hosts, efforts were made to ‘cure’ isolate 88 of its dsRNA infection by cycloheximide treatment. However, following cycloheximide treatment, a sensitive reverse transcription polymerase chain reaction (RT-PCR) amplification assay showed that the dsRNA elements, whilst being reduced in amount, were not eliminated completely and that high levels of cycloheximide also interfered with spore production, pigmentation and overall growth of the isolate. In further experiments attempts were made to mobilise the dsRNAs from 4 isolates viz. A-56, A-54, A-78 and isolate 88 into isolate Af-273y, which is hygromycin resistant and yellow in colour, by hyphal tip fusion, protoplast fusion and protoplast transfection with purified virus. Protoplast fusion and viral transfection experiments were successful for some isolates, as assessed by the RT-PCR assay and small scale extractions of nucleic acids. Subsequently comparative growth experiments by radial growth assay and mycelial weight measurements between isolate Af-273y and Af-273y transfected with isolate 88 partitivirus in essentially the same genetic background were performed. These experiments showed that the partitivirus infection resulted in a sectored phenotype and significantly lowered the growth of the fungus. All efforts to initiate the molecular characterisation of uncharacterised dsRNA elements found in isolates A-54, A-78 and isolate-66 have thus far proven unsuccessful but a new approach (cDNA library construction) is proposed for the characterisation of these dsRNAs

TGLE Vol. 52 Nos. 1 & 2 Full Issue

Full issue for TGLE Vol. 52 Nos. 1 &

The effects of non-host volatiles on habitat location by phytophagous insects

The host-plant selection sequence of phytophagous insects can be divided into three steps: (i) habitat location, (ii) host location, and (iii) host acceptance. Chemoreception plays a main role during the process. Olfaction is typically the most important sensory modality during the first two steps, whereas contact chemoreception dominates the third. At all steps in the selection sequence, positive and negative external stimuli interact with each other and also with internal factors in the insect, and the balance between positive and negative stimuli can be tipped towards either acceptance or rejection of a particular habitat or host. Non-host volatiles (NHV) have been shown to modulate host location behaviour in several insect orders. Few studies report effects at the habitat level, but in forest ecosystems, indications of anti-attractant effects at the habitat level has been found in a small number of conifer-inhabiting coleopterans and lepidopterans. In agricultural systems, intercropping studies have frequently found lower pest insect abundance in polycultures compared to monocultures. It has been hypothesized, and sometimes demonstrated, that repellent NHV from the intercrop is the main mechanism for the observed patterns of pest abundance. However, several other mechanisms have been suggested, all discussed here. Positive and negative olfactory stimuli are perceived by olfactory hairs (sensilla), mainly located on the antennae. A brief overview of the insect olfactory system is presented and encoding of positive and negative stimuli by the peripheral and central nervous system is discussed