Research Articles (Plant Pathology)

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Browsing Research Articles (Plant Pathology) by Subject "Botrytis cinerea -- Control"

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    Application of fungicides against postharvest botrytis bunch rot of table grapes in the Western Cape
    (South African Society for Enology and Viticulture, 1994) De Kock, P. J.; Holz, G.
    Fungicide programmes for the control of postharvest Botrytis bunch rot on table grapes were evaluated in six trials from 1984/85 to 1991/92 in the Western Cape. The study demonstrated the ineffectiveness of dicarboximide applications during bloom to early pea size in well managed vineyards. Dicarboximides were most effective when applied from bunch closure to ripening. lprodioue/sulphur treatments at veraison and before harvest reduced Botrytis bunch rot, but they were ineffective in inhibiting infection during storage. Control was only achieved when grapes were exposed to S02 during storage. Although bunch dip treatments reduced infection in the vineyard, this control was not commercially acceptable. Therefore no real advantage was found when bunches were dipped in fungicide at veraison to ensure better coverage. The fact that berries became infected primarily during harvest, package operations and storage, emphasised the necessity for reducing B. cinerea inoculum on harvested grapes. It is suggested that the results of this investigation may lay the foundation for incorporating biological control in Botrytis bunch rot control.
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    Fitness on grape berries of botrytis cinerea isolates belonging to different dicarboximide sensitivity classes
    (South African Society for Enology and Viticulture, 2003) Fourie, P. H.; Holz, G.
    Seasonal fluctuations in the frequency of dicarboximide-resistant Botrytis cinerea isolates in Western Cape vineyards suggest a reduced fitness of these isolates. In this study fitness of sensitive, ultra-low-level and low-level dicarboximide-resistant isolates of B. cinerea were compared on grape berries. Conidia were dispersed as single cells on berry surfaces from airborne inoculum in a settling tower or deposited as clusters in water droplets. Investigations were conducted at two potential infection courts on dicarboximide treated and untreated berries, namely the intact cuticle and wounds. Surface sterilisation, isolation and freezing techniques were used to determine surface colonisation and penetration by airborne inoculum. Decay incidence, severity and sporulation incidence were determined following inoculation with conidial clusters. The different tests indicated that germlings of dicarboximide-sensitive and -resistant isolates had similar surface-colonising abilities of dicarboximide-free berries. However, sensitive strains penetrated significantly more often. Fitness decreased with an increase in the level of dicarboximide resistance. Iprodione caused a drastic disturbance in the ratio of different dicarboximide sensitivity classes that occupied the berry surface and allowed the development of germlings of predominantly resistant isolates, but with few successful infections. Significantly higher levels of infection and proliferation of dicarboximide-resistant isolates on sprayed or unsprayed berries were facilitated by wounding or the termination of host resistance (freezing). According to these findings, these modes of infection should not contribute to a gradual build-up of inoculum of either dicarboximide-sensitive or -resistant isolates. Trends by airborne conidia described here suggest that another primary infection event in the vineyard, most likely floral infection and subsequent debris colonisation, should largely regulate the dynamics between dicarboximide-sensitive and -resistant isolates in B. cinerea populations on grapevine.
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    Fungicide efficacy against botrytis cinerea at different positions on grape shoots
    (South African Society for Enology and Viticulture, 2003) Van Rooi, C.; Holz, G.
    Fungicide efficacy at different positions on grape shoots was determined on vinelets inoculated with Botrytis cinerea. To ensure proper fungicide coverage, vinelets (table grape cultivar Dauphine, wine grape cultivar Merlot noir), prepared from cuttings, were sprayed in a spray chamber at the recommended dosages with iprodione, pyrimethanil, cyprodinil/fludioxonil and fenhexamid. Vinelets for the control treatment were left unsprayed. After 24 h the vinelets were dusted with dry, airborne conidia in a settling ower to reduce the sporadic occurrence of the pathogen on shoots and incubated for 24 h at high relative humidity (±93 % ) to promote infection. Fungicide efficacy was determined by observing intact vinelets for symptom expression at nodes, internodes, leaf blades, petioles and inflorescences, and by determining surface colonisation (shoots left unsterile) and penetration (surfacedisinfested shoots) by isolation studies. Symptoms of B. cinerea decay did not develop on sprayed and unsprayed vinelets that were kept in dry chambers during the 2-week observation period. The pathogen, however, developed from the isolated parts. On the unsprayed vinelets for both cultivars B. cinerea incidences recorded on two media and the two sterility regimes were significantly higher on leaf blades than on the petioles, shoot pieces and inflorescences. The isolation studies showed that the different fungicides were highly and nearly equally efficient in reducing superficial B. cinerea and latent infection at the various positions. In the case of leaf blades B. cinerea incidence was significantly reduced by each fungicide on both cultivars. This was not the case for the other parts, which yielded B. cinerea at low incidences under the two sterility regimes. The study showed that the fungicides used in this study, if applied properly to shoots at the pre-bloom stage, should effectively reduce the amount of B. cinerea in leaves and completely prevent the infection of nodes, internodes and inflorescences.
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    Spray deposition and control of botrytis cinerea on grape leaves and bunches : part 1 (table grapes)
    (South African Society for Enology and Viticulture, 2016) Brink, J. C.; Calitz, F. J.; Fourie, P. H.
    Insufficient quantity but also quality of spray deposition on susceptible grapevine tissue (i.e. target sites) and favourable conditions for pathogens could lead to control failure during high disease pressure situations. To determine deposition quantity and quality benchmarks for biologically effective spray deposits, bunches and leaves of table grapes (Waltham Cross) were sprayed at various growth stages, using different application volumes of a mixture of fenhexamid and a fluorescent tracer pigment and subsequently dusted with dry conidia of Botrytis cinerea where after infection levels on pedicels, receptacles and leaves were determined. Pigment deposition quantity and quality were determined from photos of sprayed parts taken with a digital camera under a stereo microscope and black light illumination at ×30 or ×10 magnifications and assessed with digital image and Hoerl regression analyses. The deposition quantity resulting in 75% control of B. cinerea infection (FPC75 values) was calculated from biological efficacy curves (sigmoidal regression analyses) for leaves and for each growth stage, for pedicels and receptacles. Deposition quantity and quality measurements correlated favourably with Botrytis infection. An optimal deposition value for the control of B. cinerea was determined by increasing spray volume, however by increasing spray volume and deposition quantity or quality levels past this optimum will not significantly improve disease control further. It was indicated that efficacy of agricultural chemicals could be influenced by improving both deposition quantity and quality, quantifiable by digital image analyses of fluorescent pigment deposition. FPC75 values obtained in this study can be used as benchmarks to evaluate future spray application in vineyards.
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    Spray deposition and control of botrytis cinerea on grape leaves and bunches : part 2 (wine grapes)
    (South African Society for Enology and Viticulture, 2016) Brink, J. C.; Calitz, F. J.; Fourie, P. H.
    Poor control of fruit and foliar diseases in vineyards is often attributed to insufficient spray deposition of susceptible tissue. To optimise spray deposition, a deposition assessment protocol using fluorometry, photomicrography and digital image analyses was developed to determine minimum spray deposition quantity and quality levels needed for effective B. cinerea control in wine grapes (Chenin blanc). Leaves and bunches were sprayed at different growth stages with different volumes of a mixture of fenhexamid and fluorescent pigment. Pigment deposition quantity and quality were determined from photos of pedicels and leaves taken with a digital camera under a stereo microscope and black light illumination at ×30 and ×10 magnification, respectively. After inoculation with dry airborne conidia of B. cinerea infection levels on pedicels, receptacles and leaves were determined and infection levels and deposition data were subjected to sigmoidal and Hoerl regression analyses, respectively. From these biological efficacy curves the deposition levels that affected 75% control of B. cinerea infection (FPC75 values) were calculated for leaves and for each growth stage for pedicels and receptacles. Deposition measurements on sprayed leaves and bunch parts correlated favourably with Botrytis infection levels. An increase in spray volume resulted in higher deposition quantity and improved quality values with a reduction of B. cinerea infections. However, at a certain point, deposition quality remained constant and infection levels did not decrease significantly with increasing spray volume. Susceptibility of pedicels and receptacles to B. cinerea decreased with maturity. FPC75 values can be used as benchmarks to evaluate spray application in wine grape vineyards.