Trevor Wicks

Trichoderma harzianum T39 and T. virens DAR 74290 as Potential Biological Control Agents for Phytophthora erythroseptica

Trichoderma harzianum isolate T39 and T. virens isolate DAR 74290 were evaluated as potential biological agents for control of pink rot of potato and root and stem rot of tomato caused by Phytophthora erythroseptica. Cell-free metabolites of T. virens DAR 74290 completely inhibited growth of P. erythroseptica in vitro and appeared to be fungicidal. T. virens DAR 74290 and Trichodex, a commercial formulation of T. harzianum T39, were tested for their ability to protect potato and tomato plants from disease caused by P. erythroseptica in glasshouse experiments. Trichodex and T. virens DAR 74290, alone and combined, reduced disease severity in shoots and roots of potatoes 10 weeks after inoculation with the pathogen. The yield of potatoes from plants treated with P. erythroseptica and T. virens DAR 74290 (mean of 12.9 g fresh weight/pot) was significantly greater than in controls inoculated with the pathogen alone (mean of 2.1 g/pot). Treatment with Trichodex alone increased the yield of tubers compared to the uninoculated controls. T. virens DAR 74290 increased the survival of tomato seedlings inoculated with the pathogen, and both this isolate and Trichodex decreased the severity of disease on tomato.

Effect of fungicide use strategies on the control of early blight (Alternaria solani) and potato yield

A series of experiments were undertaken to evaluate fungicide use strategies for the control of early blight (Alternaria solani), the most significant foliar disease of potatoes in Australia. The protective and curative activity of fungicides was evaluated in glasshouse and field studies. Boscalid, azoxystrobin and difenoconazole were highly effective in the control of early blight when applied up to three days before or three days after inoculation. Boscalid completely inhibited disease development when applied one day before inoculation. Applied five days after inoculation boscalid did not control early blight while difenoconazole was the most effective fungicide. Spray programs of 4-6 applications of fungicides from several different mode-of-action groups were evaluated in 4 field trials on commercial properties in South Australia, Western Australia and Queensland. All spray programs inhibited the development of disease and improved marketable tuber yields compared with unsprayed plots. The most effective spray programs included those with boscalid + metiram in the first two applications, and resulted in significant increases in early blight control and tuber yields of over 20%. These findings show that products with strong protective activity such as boscalid were more effective when applied earlier in the spray program. Products with strong curative activity such as difenoconazole were more effective than protective fungicides when early blight infections were well established in the crop. This information can be used to assist potato growers to improve early blight control and increase marketable tuber yields.

Mode of action of milk and whey in the control of grapevine powdery mildew

Grapevine powdery mildew, caused by the fungus Erysiphe (Uncinula) necator, is a major disease affecting grape yield and quality worldwide. In conventional vineyards, the disease is controlled mainly by regular applications of sulfur and synthetic fungicides and, in organic agriculture, by sulfur and botanical and mineral oils. Research has identified milk and whey as potential replacements for synthetic fungicides and sulfur in the control of powdery mildew. Electron spin resonance and scanning electron microscopy were used to investigate the possible mode or modes of action of milk and whey in the control of powdery mildew. Electron spin resonance experiments showed that various components of milk produced oxygen radicals in natural light, which may have contributed to the reduction of severity of powdery mildew on treated leaves. Milk and whey caused the hyphae of E. necator to collapse and damaged conidia within 24 h of treatment. Hydrogen peroxide, applied as a source of free radicals, also caused collapse of the hyphae of E. necator but did not damage conidia, and appeared to stimulate germination. Lactoferrin (an antimicrobial component of milk) ruptured conidia, but damage to hyphae was not evident until 48 h after treatment. The results support the hypothesis that free radical production and the action of lactoferrin are associated with the control of powdery mildew by milk.

Protection of grapevine pruning wounds from infection by Eutypa lata using Trichoderma harzianum and Fusarium lateritium

Abstract.Trichoderma harzianum applied to grapevine pruning wounds in a spore suspension and in the commercial formulations of Trichoseal, Trichoseal spray and Vinevax pruning wound dressing reduced recovery of Eutypa lata in the glasshouse and in the field. Recovery of E. lata was significantly reduced (P < 0.001) when fresh wounds were treated with viable T. harzianum 2 or 7 days before inoculation with ascospores of the pathogen in the glasshouse. In field experiments, recovery of E. lata was significantly reduced (P < 0.001) when fresh pruning wounds were treated with spores of T. harzianum, Fusarium lateritium or Vinevax 1 or 14 days before ascospores were applied. In general, a delay of 14 days between wounding and inoculation with ascospores of E. lata reduced recovery of the pathogen compared with inoculation on the day after wounding.

The Influence of Grapevine Cultivar and Isolate of Eutypa lata on Wood and Foliar Symptoms

Grapevine cultivar (Vitis vinifera) and isolate of Eutypa lata influence wood and foliar symptoms of Eutypa dieback. Foliar symptoms of Eutypa dieback developed within 8 months of inoculating young grapevines (cvs. Grenache, Cabernet Sauvignon, and Merlot) in a shadehouse. Isolates of E. lata from various wine regions in southern Australia varied in their ability to colonize inoculated grapevines and induce wood and foliar symptoms. Grapevine cultivars varied for wood and foliar symptom expression but not for mycelial colonization. However, the severity of foliar symptoms was not related to the rate of spread of the fungus in the grapevine. Furthermore, the staining of wood typically attributed to E. lata did not reflect the presence of the fungus because the fungus was detected up to 80 mm beyond the stain. A field trial with mature grapevines revealed significant differences in the rate of spread of wood staining due to E. lata among eight cultivars, with up to 50 mm/year detected in Cabernet Sauvignon and Shiraz grapevines. In the shadehouse, the maximum growth rate of E. lata was recorded to be 115 mm/year for Grenache rootlings. Information from this study may help to optimize management strategies for maintaining productivity of grapevines with Eutypa dieback, thus reducing the economic impact of the disease.

Reduced sensitivity of Uncinula necator to sterol demethylation inhibiting fungicides in southern Australian vineyards

Isolates of Uncinula necator with reduced sensitivity to triadimenol and, to a lesser extent, fenarimol have been detected in Australian vineyards. Grapevine leaves and berries affected by powdery mildew were collected from a home garden and 13 vineyards in nine viticultural regions in Australia between 1993 and 1998. Single-spore isolates of U. necator established from vines, either not exposed to DMIs (‘unexposed’ collection) or treated with DMIs (‘selected’ collection), were maintained on micro propagated grapevines in vitro. A bioassay for fungicide sensitivity was used to test 60 single-spore isolates of U. necator for sensitivity to triadimenol. Of these, 34 were tested for sensitivity to fenarimol. Mean EC50 values for the 12 unexposed isolates were 0.07 and 0.08 mg/L for triadimenol and fenarimol, respectively. For the selected isolates, mean EC50 values were 0.83 mg/L for triadimenol and 0.19 mg/L for fenarimol. In comparison with the unexposed population, there appeared to have been a shift in the selected population towards reduced sensitivity. Examination of more isolates from vineyards not exposed to DMIs may strengthen this conclusion. Cut-off EC50 values, used to define individual isolates as having reduced sensitivity, were 0.42 mg/L (resistance factor, RF = 6) for triadimenol and 0.12 mg/L (RF = 1.5) for fenarimol. The findings were consistent with the existence of cross-resistance between triadimenol and fenarimol.

Fungicidal control of Phytophthora erythroseptica: the cause of pink rot on potato.

Two shade house and six field experiments were undertaken to evaluate fungicides and timing of application for the control of pink rot of potatoes caused byPhytophthora erythroseptica. Pink rot developed in up to 60% of tubers grown in artificially inoculated soil and 21% of the tubers in naturally infected soil. Ridomil (metalaxyl) and Ridomil Gold (mefenoxan) granules applied to the soil at planting completely or nearly completely controlled pink rot throughout the season in both pot and field experiments. Foliar applications of Ridomil Gold MZ (mefenoxan plus mancozeb) applied when tubers were around 20 mm diameter and another one or two sprays 12 to 28 days later also provided more than 80% control of pink rot in most experiments. Foliar applications of Shirlan (fluazanim) or Acrobat MZ (dimethomorph plus mancozeb) showed some activity against pink rot, but were significantly less effective than Ridomil Gold MZ. Foli-R-Fos 200 (phosphonic acid) or Dithane M45 (mancozeb) applied to the canopy showed little activity against pink rot in most experiments. Yield increases of between 6 and 17 tons ha−1 resulted from the application of either Ridomil granules at planting or two to three Ridomil Gold MZ foliar sprays around “tuber set.”ResumenDos experimentos en sombráculo y seis en el campo fueron emprendidos para evaluar fungicidas, y el cronometraje de su aplicación a fin de controlar la pudrición rosada de la papa causada porPhytophthora erythroseptica. La pudrición rosada se desarrolló en el 60% de los tubérculos cultivados en suelo artificialmente inoculado, y en el 21% de los tubérculos en suelo naturalmente infectado. Gránulos de Ridomil (metalaxyl) y Ridomil Gold (mefenoxan), aplicados al suelo al sembrar, controlaron totalmente, o casi totalmente, la pudrición rosada durante la temporada, tanto en los experimentos de maceta, como en los del campo. Las aplicaciones foliares de Ridomil Gold MZ (mefenoxan más mancozeb) hechas cuando los tubérculos tuvieron diámetros de cerca de 20 mm, y otras 1 o 2 pulverizaciones 12 a 28 días después, también proveyeron más del 80% del control de la pudrición rosada en la mayoría de los experimentos. Las aplicaciones foliares de Shirlan (fluazsnim), o Acrobat MZ (dimethomorph más mancozeb), mostraron cierta actividad contra la pudrición rosada, pero fueron considerablemente menos efectivas que Ridomil Gold MZ. Foli-R-Fos 200 (ácido fosfónico) o Dithane M45 (mancozeb), aplicada al dosel, mostró poca actividad contra la pudrición rosada en la mayoría de los experimentos. Aumentos de rendimiento de entre 6 y 17 toneladas ha−1 resultaron de la aplicación o de gránulos de Ridomil al sembrar, o de 2 a 3 pulverizaciones foliares de Ridomil Gold MZ cerca del asentamiento de los tubérculos.

The influence of climate on foliar symptoms of eutypa dieback in grapevines

ABSTRACT Foliar symptoms of Eutypa dieback, caused by Eutypa lata, in grapevines, cv. Shiraz, varied from year to year in a 6-year study conducted in South Australia and, although trends were similar for vineyards within geographical regions, differences were observed between regions. We attempted to elucidate the causes underlying this variation and hypothesized that it was influenced by climatic factors. A number of possible relationships were identified between climate and symptom expression: (i) increased symptom expression was related to increased winter rainfall 18 months earlier, (ii) decreased disease incidence and prevalence were related to increased temperature in spring, and (iii) a reduction in disease incidence was related to both very high and very low rainfall in October. Theories for these relationships are proposed and require further investigation. A conceptual model was developed which requires validation and has the potential to predict the incidence of foliar symptoms of Eutypa dieback. Information from this study could lead to an improved integrated pest management system to suppress foliar symptoms and sustain productivity of vines infected with E. lata.

Interaction between Eutypa lata and Trichoderma harzianum

Metabolites produced by three strains of Trichoderma harzianum reduced growth of Eutypa lata in vitro. Volatile metabolites produced by T. harzianum were fungistatic towards both isolates of E. lata tested. Growth of some isolates of E. lata was inhibited completely by non-volatile metabolites. Infection by E. lata was reduced in autoclaved grapevine cane segments co-inoculated with spores of T. harzianum and E. lata. Scanning electron microscopic examination of gamma-irradiated cane segments and living cuttings inoculated with T. harzianum and E. lata suggested that antagonism in grapevine wood was mainly by antibiosis. Both the pathogen and the antagonist grew in the xylem vessels and pith parenchyma cells of the wood.

The incidence of Alternaria radicina on carrot seeds, seedlings and roots in South Australia

Surveys of eight carrot growing regions in South Australia conducted from December 1998 to May 1999, showed that poor seedling establishment was associated with high levels of infection by Alternaria radicina. Seedling losses were widespread throughout the state and were most frequent between February and April 1999. Up to 47% of seedlings and 88% of mature plants on some properties were infected by A. radicina. Isolations from diseased plants showed that A. radicina attacked carrots at all stages, causing damping-off, and rotting of roots, crowns, seedlings, petioles, leaves and crowns of maturing carrots. Up to 70% of mature carrots with crown and shoulder infections were unmarketable. Of the 19 lines of commercial seed tested, 18 were infected with A. radicina with the highest infection level of 35% occurring on imported seed. A. radicina at levels between 0.2 and 14% was also found in 11 of 16 seed batches treated with thiram and/or iprodione. Isolates of A. radicina from seeds or infected seedlings were pathogenic to carrot seedlings and carrot discs. Alternaria dauci was also isolated from two seed samples, with levels of infection ranging from 0.1–0.3%, and on foliage in three of the eight carrot growing regions.