SUNScholar - Browsing by Author "Holz, G."

Browsing by Author "Holz, G."

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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.
Cleistothecia and flag shoots : sources of primary inoculum for grape powdery mildew in the Western Cape Province, South Africa
(South African Society for Enology and Viticulture, 2000) Halleen, F.; Holz, G.
Little is known about the mode of survival and sources of primary inoculum of Uncinula necator, the causal pathogen of grapevine powdery mildew, in vineyards in the Western Cape province. A study was therefore undertaken to determine whether cleistothecia and flag shoots are formed on vines in local vineyards. Flag shoots were found shortly after budbreak in September 1997 in a Carignane vineyard near Somerset West. Cleistothecia were first observed during April to May 1996 on severely infected leaves from three vineyards in the main grapegrowing areas of Stellenbosch. This was the first report of cleistothecia and flag shoot formation in vineyards in the Western Cape province. Cleistothecia occurred in small numbers on leaves (1 - 10 per leaf) and all were immature. Cleistothecia were dispersed by late summer and autumn rains from leaves to bark of grapevines, where they overwinter. No conclusion could be made regarding the viability of cleistothecia. However, the characteristics of the first symptoms that developed on leaves, namely separate, individual lesions formed at random on first-formed leaves growing in close proximity to the bark, provided circumstantial evidence that cleistothecia are dispersed to the bark. Weather conditions suitable for release of ascospores from overwintered cleistothecia occurred frequently between budbreak and bloom in all the areas.
Colonization of table grape bunches by alternaria alternata and rot of cold-stored grapes
(South African Society for Enology and Viticulture, 1994) Swart, A. E.; Holz, G.
Assessment of latent Alternaria alternata infections in table grapes indicated that infections occurred in commercial vineyards during the entire period of bunch development. Mature bunches were asymptomatic despite high levels of A. alternata recovered from triple-sterilized bunch tissue. Inoculation studies showed no shift in disease susceptibility of ripening grape berries, and postharvest rot was not related to the level of natural infection. Late season fungicidal sprays, or dip treatments that ensured better penetration and coverage of inner parts, resulted in no meaningful reduction in postharvest rot. Based on the behaviour of the pathogen, it is suggested that additional fungicide programmes for the control of the disease should not be followed in commercial vineyards. Instead, attention should be given to physiological and stress factors, such as mechanical and sulphur dioxide damage that might predispose cold-stored bunches to A. alternata decay.
Effect of fungicide spray cover on botrytis cinerea infection in grape bunches
(South African Society for Enology and Viticulture, 2006) Brink, J. C.; Holz, G.; Fourie, P. H.
The effect of fungicide spray cover on Botrytis cinerea infection in grape bunches was determined. Bunches were sprayed at pea size and bunch closure with different volumes of a mixture of fenhexamid and a yellow fluorescent pigment, and the percentage fluorescent pigment coverage on pedicels was determined. Bunches were subsequently dusted with dry airborne conidia of B. cinerea in a settling tower and incubated for 24 h at high relative humidity (98%). Infection was determined by estimating the amount of B. cinerea infections on susceptible bunch parts (pedicel, receptacle and rachis) with isolations onto paraquat (herbicide) and Kerssies (B. cinerea selective) mediums. Linear regressions for the part × stage combinations of percentage B. cinerea incidence on different bunch parts were fitted on mean coverage levels. An increase in spray cover caused linear reductions in levels of B. cinerea on bunch parts. Higher B. cinerea incidences were recorded at pea size. Furthermore, higher B. cinerea incidences were found on paraquat medium for both stages, than on Kerssies medium. The information gathered from this study will be used to facilitate future determination of minimum effective coverage levels for effective B. cinerea control in grape bunches.
Epidemiology of botrytis cinerea on grape : wound infection by dry, airborne conidia
(South African Society for Enology and Viticulture, 2002) Coertze, S.; Holz, G.
This study describes the infection of fresh wounds on berries exposed to freshly deposited airborne Botrytis cinerea conidia, and on berries carrying previously deposited conidia and germlings (latent infections). Grapes at bunch closure and at the mature (harvest) stage, as well as mature, cold-stored grapes, were used. The grapes were dusted with dry conidia in a settling tower. The inocula were subjected to conditions commonly encountered by the pathogen in grape bunches: dry conidia on dry berries under dry conditions, dry conidia on dry berries under high relative humidity, and dry conidia exposed to a film of water on the berry surface. The mean number of wounds that yielded B. cinerea decay 14 days after inoculation was calculated. Fluorescence microscopy revealed that conidia occurred evenly as single cells on the grape berry surface and seldom landed at the wound periphery. They remained dormant on dry berries, but germinated freely on the unbroken skin and at the wound periphery on moist and wet berries. In the case of berries inoculated at bunch closure and harvest stages, wounds were not infected by conidia deposited on berries four days prior to wounding. This finding indicated that, following adhesion and the first stages of growth, the pathogen did not survive for extended periods on surfaces of immature and mature grape berries. Freshly deposited dry conidia were needed to infect the wounds. The freshly deposited conidia furthermore needed free water, and not high humidity or wound exudates, to infect the fresh wounds. Proportions of wounds infected were extremely low. According to these findings, this mode of infection should not contribute to a gradual build-up of secondary inoculum and to B. cinerea epiphytotics in the vineyard. The previously and freshly deposited conidia both infected wounds made on cold-stored mature berries. Of the two inocula, freshly deposited conidia were more successful in causing wound infection. Therefore, in the event of wounding, postharvest decay would be caused primarily by conidia occurring on the grape berry surface, and not by mycelia in the berry skin.
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.
Frequency of dicarboximide resistant strains of botrytis cinerea in South African table grape vineyards and influence of spray schedules on resistant sub-populations
(South African Society for Enology and Viticulture, 1998) Fourie, P. H.; Holz, G.
Between 1993 and 1995, 1139 isolates of Botrytis cinerea were collected from table grape vineyards in the Western and Northern Cape provinces and tested for sensitivity to dicarboximides (vinclozolin) and benzimidazoles (carbendazim). Overall the frequency of resistance to vinclozolin and carbendazim averaged 9,4 and 11,9%, respectively. Ninety-seven percent of the vinclozolin-resistant isolates were dual resistant to carbendazim. Fifty-nine percent of the isolates were from the Hexriver region; of these 7,7% were vinclozolin, and 12,6% carbendazim resistant. The Paarl region had the highest resistance incidence (37,7% vinclozolin and 41,5% carbendazim resistant). Only two of the 191 isolates tested from the Orange River region were vinclozolin resistant. ED50 values of ultra-low-level and low-level dicarboximide-resistant isolates ranged from 0,843 to 1,834 and 2,053 to 5,129 μg vinclozolin/ml, respectively. Vinclozolin-resistant strains were not abnormally osmotically sensitive. Monitoring of changes in the frequency of resistance in 10 commercial vineyards under high-, medium- and low-schedule dicarboximide programmes from 1993-1995 showed that the resistance frequency fluctuated from low (average 12,7% ) during the winter to high (average 55,8 % ) during the growing season. Maximum levels of dicarboximide resistance were recorded during bunch closure. Distinct differences were, however, observed in the resistance frequency in individual vineyards. In four high-schedule dicarboximide vineyards resistance frequencies increased early in the season, prior to any dicarboximide applications. This finding suggests dual resistance between broad-spectrum fungicides and dicarboximides.
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.
Incomplete cross-resistance to folpet and iprodione in botrytis cinerea from grapevine in South Africa
(South African Society for Enology and Viticulture, 2001) Fourie, P. H.; Holz, G.
The sensitivity to folpet of Botrytis cinerea isolates obtained from table grape vineyards in the Western Cape province of South Africa with a known history of dicarboximide (DC) resistance and high-schedule DC and folpet programmes was investigated. In the Simondium vineyards, 61 % of the B. cinerea isolates from Dan-hen-Hannah and 20% of the isolates from Waltham Cross were resistant to iprodione. In the Northern Paarl vineyards, 95% of the isolates from Dan-hen-Hannah and 95% of the isolates from Waltham Cross were designated resistant. In the case of the iprodione-sensitive isolates from vineyards in Simondium, folpet ECso values ranged from 4.9 to 29.1 μg/mL for the Dan-hen-Hannah and 15.0 to 43.5 μg/mL for the Waltham Cross sub-populations, respectively. Folpet ECso values of the iprodione-resistant isolates, on the other hand, ranged from 19.7 to above 100 μg/mL for the Dan-hen-Hannah subpopulation. In the Northern Paarl subpopulations, where the isolates were predominantly iprodione-resistant, folpet ECso values of the latter isolates ranged from 21.5 to above 100 μg/mL. Similar shifts in folpet sensitivity were displayed by ultra-low- and low-level DC-resistant B. cinerea isolates obtained from other regional subpopulations. The results indicated incomplete cross-resistance between iprodione and folpet. This finding suggests that early increases in DC resistance frequencies in B. cinerea, observed prior to DC application in vineyards under the high-schedule DC and folpet programmes, can be attributed to incomplete cross-resistance to these fungicides in sub-populations of the pathogen.
Infection of table grape bunches by alternaria alternata
(South African Society for Enology and Viticulture, 1995) Swart, A. E.; Lennox, C. L.; Holz, G.
Infection of table grape bunches by Alternaria alternaJa was investigated by light, fluorescence and scanning electron microscopy. Conidia in 20-μ1 drops of spore suspension germinated readily on the fruit surface and within 16 h grew extensively on berries, pedicels and rachises of immature and mature bunches. Appressoria were formed within 16 hat the tip of germ tubes and hyphae, and on short side branches on the different bunch parts, but no evidence of direct penetration was found. The pathogen penetrated the host tissue through stomata, lenticels and microcracks in the epidermis. Infection hyphae remained localized in the substomatal cavities or surface cells oflenticels, and were restricted to a few epidermal cells in the case of epidermal microcracks, but caused no cell necrosis. Under conditions of high humidity, the fungus grew out of the stomata, lenticels and microcracks and formed an extensive superficial growth within 168 h. Although the fungus was able to grow into epidermal cells adjacent to those surrounding wounds, it would appear that this process is a slow one. The behaviour of A. alternala partly explains the extensive superficial growth of the pathogen that may occur on rachises and pedicels of table grapes at the end of the cold storage period.
Occurrence of fruit-decaying fungi on adult male Mediterranean fruit flies (ceratitis capitata) captured in orchards and adjacent vineyards
(South African Journal of Enology & Viticulture, 2004) Engelbrecht, R.; Holz, G.; Pringle, K. L.
The occurrence of adult male Ceratitis fruit flies and their potential to transmit fungi associated with pre- and postharvest decay of fruit in natura were investigated. Sensus fruit-fly traps were installed in orchards each bordering on a vineyard on farms in the Stellenbosch region, South Africa. Ceratitis fruit flies were collected weekly, identified and counted to determine the fluctuations in fruit-fly population levels. Captured fruit flies were plated on a semi-selective medium and the number of flies yielding the fungi was recorded. Both the Mediterranean fruit fly (C. capitata), and the Natal fruit fly (C. rosa) were trapped. C. rosa seldom occurred and comprised only 1 % of the total number of flies captured. Fruit-fly patterns showed that early infestation of orchards may contribute to the infestation of adjacent vineyards later in the season. At localities where flies were not trapped in the early season, infestation usually occurred in the orchards and adjacent vineyards in the late season, and well after fruits were harvested. Fruit flies from all localities yielded species of Alternaria, Penicillium, Aspergillus, Rhiwpus, Mucor and Botrytis cinerea on the medium. Of the different fungi, Alternaria and Penicillium spp. were most commonly carried by the flies at the various localities. The organisms occurred in a specific pattern on flies captured at a specific locality, and in a given orchard or vineyard. The pattern of fruit-fly infestation and their fungal contamination suggest that the Mediterranean Fruit-fly has the potential to transfer fungi associated with post-harvest decay in natura from early-season to mid- and late-season peach and plum orchards, and to neighbouring mid- and late-season wine and table grape vineyards. This highlights the importance of fruit-fly management in order to control disease epidemics in orchards and vineyards.
An overview of the biology, epidemiology and control of uncinula necator (powdery mildew) on grapevine, with reference to South Africa
(South African Society for Enology and Viticulture, 2001) Halleen, F.; Holz, G.
Grape powdery mildew, caused by Uncinula necator, is the most widespread and destructive disease of grapevine. The disease can be found in most grape-growing areas of the world, including the tropics. In South Africa, grape powdery mildew was first reported in 1880, and since then has become the most important disease of grapevine. The disease can effect all phases of plant growth, and without necessarily causing obvious symptoms, may have a harmful effect on the vine and its products. The pathogen follows a specific pattern in each part of the world to create an epidemic. This pattern is determined by biological characteristics of the organism, climatic factors, cultivation practices and cultivar choices. Increased world-wide emphasis on the production of disease-free grapes with minimal fungicide input provides a sound reason for exploring more efficient disease management strategies through a better understanding of U. necator epidemiology and population genetics. Knowledge of these aspects is available for various parts of the world, but little is known about its relevance to South African vineyards. In this article a South African perspective of the pathogen and its control is outlined, based on recent local findings, and considered in the light of knowledge available in other parts of the world.
Resistance in uncinula necator to triazole fungicides in South African grapevines
(South African Society for Enology and Viticulture, 2000) Halleen, F.; Holz, G.; Pringle, K. L.
The distribution of Uncinula necator variants resistant to triadimenol, penconazole and flusilazole were determined in vineyards with suspected resistance in the regions Tulbagh, Franschhoek, Stellenbosch, De Dooms, Riebeeck Kasteel and Paarl. The regional subpopulations had all been exposed to triadimefon or triadimenol prior to 1989, when these fungicides were phased out and other demethylation-inhibiting fungicides (DMls) were applied. The occurrence of resistant variants in the subpopulations was compared with those in a vineyard in the Ceres Karoo region, which was isolated by two mountain ranges from the viticultural regions and where triadimefon was used prior to 1989, then abandoned. No other DMls were applied. A discriminatory germ tube length was used as a criterion to distinguish between sensitive and tolerant conidia at a discriminatory fungicide dose of 0.3 μg/mL. All the populations showed reduced sensitivity to triadimenol. This finding indicated an earlier shift in triadimenol sensitivity in the subpopulations and showed that resistant variants are sufficiently competitive to become established in vineyards. Cross-resistance between the triazoles was indicated by the frequency at which resistant variants occurred in subpopulations. The Ceres Karoo population was at baseline sensitivity level for penconazole and flusilazole. However, the four populations (De Dooms, Franschhoek, Riebeeck Kasteel and Stellenbosch) which showed the highest shifts in sensitivity to triadimenol, also displayed a high level of reduced sensitivity to flusilazole. This was in spite of the fact that only the Stellenbosch population was regularly treated with flusilazole. The other three populations were predominantly exposed to penconazole. Reduced sensitivity to penconazole was furthermore most prevalent in the Paarl K, Paarl I, Riebeeck Kasteel and De Dooms populations. Of these populations, Paarl K and Paarl I received predominantly penconazole, whereas the other two populations were treated with a range of DMis. Penconazole EDso values for the Paarl K, Paarl I, Riebeeck Kasteel and De Dooms pathogen populations (which showed the highest shifts in sensitivity to this fungicide) were 0.908, 1.022, 1.253 and 1.942 μg/mL, respectively. In these populations, 53%, 38%, 71 % and 91 % of the conidia respectively belonged to the 1.0-3.0 μg/mL and higher resistant classes. Reduced sensitivity to flusilazole was most prevalent in the Stellenbosch, De Doorns, Riebeeck Kasteel and Franschhoek populations. Flusilazole EDso values for these populations were 1.580, 1.813, 2.143, 3.885 μg/mL, respectively, whereas 83%, 82%, 96% and 79% of the conidia respectively belonged to the 1.0-3.0 μg/mL and higher resistant classes. These findings suggest a differing sensitivity of the pathogen to the three triazole fungicides which indicate that resistance to DMis is a multigenic trait in U. necator.
Use of gamma irradiation for control of postharvest botrytis cinerea bunch rot of table grapes in cold storage
(South African Society for Enology and Viticulture, 1991) De Kock, P. J.; Holz, G.
The feasibility of employing gamma irradiation for the control of postharvest Botrytis bunch rot of table grapes in cold storage was studied. Table grape cultivars from commercial vineyards in the Paarl and Hex River Valley areas were packed as for export in vented corrugated cartons. The cartons were irradiated at O; 1,5; 2,0 or 3,0 kGy. After irradiation, grapes were kept for 4 weeks at -0,5°C, followed by another week at 10°C. Less decay was observed when table grapes were irradiated soon after packing than after a storage period. Irradiation at 1,5; 2,0 and 3,0 kGy reduced the effect of Botrytis cinerea on Barlinka grapes stored without sulphur dioxide. Irradiation of Waltham Cross and Barlinka grapes at a dose of 2,0 kGy, combined with reduced S02 treatments, resulted in similar control as with the standard practice of enclosing an S02 generator. Browning of Waltham Cross berries and bacterial and yeast growth occurred on the surface of berries irradiated at a dose of 3,0 kGy. Irradiation had no adverse effect on other aspects of quality.

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