Frikkie J. Calitz

Pathogenicity of three Fusarium species associated with asparagus decline in South Africa

Isolations were made from crown, root, and stem lesions of symptomatic U.C. 157 F2 asparagus plants and from soil debris sampled in a declining asparagus field. Fusarium oxysporum, F. proliferatum, and to a lesser extent F. solani were the three dominant fungi isolated. Isolates of each species were evaluated for pathogenicity by inoculating U.C. 157 F2 plants in a greenhouse assay and an in vitro agar-test-tube assay. All three Fusarium spp. were pathogenic to asparagus, although they differed significantly in their disease-causing ability. In an in vitro assay, F. proliferatum isolates had a mean disease rating class of 4 compared with a class 3 for F. oxysporum and 2 for F. solani, on a scale of 1-5. Significant differences between F. proliferatum isolates were also detected. The high frequency of isolation and virulence of both F. oxysporum and F. proliferatum indicate that both species are important pathogens associated with asparagus decline in South Africa

Effect of crop rotation on crown rot and the incidence of Fusarium pseudograminearum in wheat in the Western Cape, South Africa

The effect of different crop rotation treatments and within-treatment crop sequences on crown rot and the incidence of Fusarium pseudograminearum in crowns of wheat from a long-term crop rotation experiment at Langgewens Experimental Farm, Moorreesburg, South Africa, were studied in 2000, 2001 and 2002. Crop rotation treatments included the following crops: canola (C), lupin (L), medic (M), medic-clover mixture (Mc) and wheat (W). Rotation treatments (RT) and within-treatment crop sequences were: RT 1=W-W-W-W, RT 2=C-W-W-W, W-C-W-W,W-W-C-W, RT 3=C-W-L-W, L-W-C-W, RT 4=W-L-C-W, L-C-W-W, RT 5=M-W-M-W, RT 6=Mc-W-Mc-W and RT 7=M-C-M-W. Crop rotation significantly affected the incidence of F. pseudograminearum, crown rot incidence and severity, dry mass and grain yield. The incidence of F. Pseudograminearum was significantly higher on wheat for the rotation treatment that included 3 years of wheat and 1 year of canola compared with the other treatments, and the highest severities and incidences of crown rot and the lowest yields were also recorded for this treatment and the monoculture wheat. The lowest incidences of the fungus, as well as crown rot incidences and severities, and the highest yields were recorded for the rotation treatment that alternated wheat with a medic-clover mixture and the treatment that included wheat after 3 years of rotation with broadleaf crops. Crop rotation should, therefore, be an important component of management strategies against crown rot of wheat.

Relationship Between Microclimatic Data, Aroma Component Concentrations and Wine Quality Parameters in the Prediction of Sauvignon blanc Wine Quality

Sauvignon blanc grape chemical and wine sensory data, as well as meteorological data (temperature and visible light radiation), collected in three climatically different wine regions in South Africa over three seasons and from two different canopy treatments were statistically analysed. A model for the prediction and/or definition of Sauvignon blanc wine quality was developed. The model utilises above- and within-canopy radiation and can explain 68.8% of the variation in the cultivar-typical vegetative/asparagus /green pepper intensity of Sauvignon blanc wine. Other significant correlations, e.g. between temperature and monoterpene concentrations, were also obtained. Further research is necessary to test and refine this model for application under different environmental conditions.

Pythium spp. Associated with Rooibos Seedlings, and Their Pathogenicity Toward Rooibos, Lupin, and Oat

Rooibos (Aspalathus linearis) is an important indigenous crop in South Africa. Oomycetes are a common problem in rooibos nurseries, causing serious losses, but limited information is available on the species involved. Molecular and morphological analyses of 117 oomycete isolates from 19 rooibos nurseries and 33 isolates from 11 native rooibos sites revealed the presence of several Pythium spp., including Pythium acanthicum, P. irregulare, P. mamillatum, P. myriotylum, P. pyrilobum, P. cederbergense, and Pythium RB II, and Phytophthora cinnamomi (native site). Most of the species were identified in nurseries and native rooibos, with Pythium irregulare being the most common species occurring in all nurseries and 46% of the native sites. Phylogenetic analyses of the internal transcribed spacer region of the P. irregulare isolates showed that isolates within this species complex fit into three subclades, of which only two have previously been reported. On rooibos, all species except P. acanthicum and the previously characterized P. cederbergense and Pythium RB II were pathogenic and highly virulent. On lupin and oat, rotation crops in nurseries, the three aforementioned species were also nonpathogenic. All the other oomycete species were pathogenic on lupin but less so than on rooibos. On oat, only P. irregulare, P. myriotylum, and P. pyrilobum were pathogenic. This is the first report of P. mamillatum, P. pyrilobum, and P. myriotylum as pathogens of lupin, and P. irregulare and P. pyrilobum as pathogens of oat. The three nonpathogenic Pythium spp. were able to significantly reduce disease caused by pathogenic species in the less susceptible lupin and oat but not on rooibos. On lupin, the nonpathogenic species enhanced the virulence of Phytophthora cinnamomi.

Occurrence of Botrytis cinerea and Monilinia laxa on nectarine and plum in Western Cape orchards, South Africa

The occurrence of grey mould (caused by Botrytis cinerea) and brown rot (caused by Monilinia laxa) in the major stone fruit regions in the Western Cape Province was studied over a 3-year period by sampling reject fruit from the Unifruco Quality Evaluation Scheme and from 11 stone fruit orchards. Flowers and fruit from the orchards were treated to facilitate disease expression by surface conidia and latent mycelia (unsterile vs. surface sterilised; untreated vs. paraquat-treated). B. cinerea was found to be the most important pathogen causing blossom blight and postharvest decay on stone fruit. The pathogen was most prominent on early and mid-season cultivars. Surface conidia and latent mycelia consistently occurred on fruit in each orchard, although at fluctuating levels. The amount of B. cinerea on fruits was generally higher during spring than summer. Disease expression on fruit was not governed by the amount of B. cinerea occurring on fruit, but by the ability of fruit to resist disease expression. Blossom infection did not contribute directly to postharvest grey mould. Brown rot was exclusively caused by M. laxa and no evidence was found that M. fructicola had been introduced into the Western Cape Province of South Africa. The pathogen was most prominent on the later maturing cultivars. Immature fruit were generally pathogen-free and disease expression only occurred on maturing fruit. Long-term latency therefore does not seem to play a prominent role in M. laxa fruit rot.

Evaluation of strategies for the control of canola and lupin seedling diseases caused by Rhizoctonia anastomosis groups

Several methods with potential for the management of Rhizoctonia diseases of canola and lupin including plant resistance, fungicide seed treatment and biological control using binucleate Rhizoctonia anastomosis groups (AGs) were evaluated under glasshouse conditions. Screening included the examination of resistance of eight canola and eight lupin cultivars/selections to damping-off and hypocotyl/root rot caused by the multinucleate Rhizoctonia solani AG-2-1, 2–2, 4 and 11. All canola cultivars were highly susceptible to AG-2-1, but Rocket, Spectrum and 44C11 were more resistant than the other cultivars. Spectrum and 44C73 were also more resistant to AG-4 than the other canola cultivars. On lupin, R. solani AG-2-2 and 4 were most virulent, and the cultivar Cedara 6150 and selection E16 were most resistant to AG-2-2; Cedara 6150, E16, Mandelup and Quilinock were more resistant to AG-4 than the other cultivars/selections. The Lupinus luteus selections, E80.1.1.2 and E82.1.1 were most susceptible to AG-2-2, 4 and 11. Seed treatment with the fungicides Cruiser OSR (a.i. difenconazole, fludioxonil, metalaxyl-M, thiamethoxam) and SA-combination (a.i. iprodione, metalaxyl, thiram) significantly increased survival of canola and lupin seedlings, decreased hypocotyl/root rot and improved the percentage of healthy seedlings, with the SA-combination being significantly more effective than Cruiser OSR. Application of the binucleate Rhizoctonia AGs (A, Bo, K and I) significantly increased the survival of lupin seedlings inoculated with R. solani AG-2-2 and 4, and AG-I and K significantly improved survival of canola in the presence of AG-4. This is the first report of the potential of binucleate AGs to protect canola and lupin seedlings against infection by multinucleate AGs.

Suppression of Pythium and Phytophthora Damping-Off of Rooibos by Compost and a Combination of Compost and Nonpathogenic Pythium Taxa

Pathogenic oomycetes, including Phytophthora cinnamomi and several Pythium spp. (Pythium irregulare, P. mamillatum, P. myriotylum, and P. pyrilobum), cause serious damping-off problems in rooibos (Aspalathus linearis) nurseries. The management of these pathogens in organic nurseries is problematic, because phenylamide fungicides may not be used. Compost, or compost in combination with Pythium taxa that are nonpathogenic to rooibos (P. acanthicum, P. cederbergense, and Pythium RB II), were investigated as alternative management options. Compost was able to suppress damping-off caused by several oomycete isolates but there was within- and between-species variation among the 30 evaluated isolates. This phenomenon was observed using two compost batches (A and B) sourced from independent suppliers. Compost B significantly reduced damping-off caused by 60% of the isolates, whereas compost A controlled only 37% of the isolates. The pathogens that were more readily controlled by both composts included P. mamillatum and P. pyrilobum, whereas the composts were ineffective at suppressing damping-off caused by >62% of P. irregulare and >50% of P. myriotylum isolates. Based on the evaluation of one Phytophthora cinnamomi isolate, this pathogen may also be controlled by compost. Neither of the composts as a stand-alone treatment could suppress damping-off caused by a combination of pathogenic species (P. cinnamomi, Pythium irregulare, P. mamillatum, P. myriotylum, and P. pyrilobum). However, damping-off was significantly reduced when nonpathogenic Pythium taxa (P. acanthicum, P. cederbergense, and Pythium RB II) were combined with the composts. Similarly, damping-off caused by a P. irregulare isolate that was not suppressed by either of the composts alone was significantly suppressed when the two composts were inoculated with the nonpathogenic Pythium taxa.

Effect of cropping system on composition of the Rhizoctonia populations recovered from canola and lupin in a winter rainfall region of South Africa

Rhizoctonia spp. anastomosis groups (AGs) associated with canola and lupin in the southern and western production areas of the Western Cape province of South Africa were recovered during the 2006 and 2007 growing seasons and identified using sequence analyses of the rDNA internal transcribed spacer regions. The effect of crop rotation systems and tillage practices on the recovery of Rhizoctonia spp. was evaluated at Tygerhoek (southern Cape, Riviersonderend) and Langgewens (western Cape, Moorreesburg) experimental farms. Isolations were conducted from canola planted after barley, medic/clover mixture and wheat, and lupin planted after barley and wheat, with sampling at the seedling, mid-season and seedpod growth stages. In the 2006 study, 93.5% of the Rhizoctonia isolates recovered were binucleate and 6.5% multinucleate; in 2007, 72.8% were binucleate and 27.2% were multinucleate. The most abundant AGs within the population recovered included A, Bo, I and K, among binucleate isolates and 2-1, 2-2 and 11 among multinucleate isolates. Crop rotation sequence, tillage and plant growth stage at sampling all affected the incidence of recovery of Rhizoctonia, but certain effects were site-specific. The binucleate group was more frequently isolated from lupin and the multinucleate group from canola. AG-2-1 was only isolated from canola and AG-11 only from lupin. This study showed that important Rhizoctonia AGs such as AG-2-1, 2-2 and 11 occur in both the southern and the western production areas of the Western Cape province and that crop rotation consistently influences the incidence and composition of the Rhizoctonia community recovered from the cropping system.

The Effect of Ethephon on the Berry Colour of Flame Seedless and Bonheur Table Grapes

Flame Seedless grafted onto Richter 99 situated in the Lower Orange River region and Bonheur grafted onto Ramsey in the Berg River Valley region of South Africa were used to test ethephon treatments for berry colour improvement. The trial was repeated over three seasons. Four ethephon dosages (100, 200, 300 and 400 mg/L) in combination with five application times (weekly intervals starting from acid maximum) were compared to an untreated control. As soon as general minimum maturity for export was reached, pack-out percentage was determined and representative bunch samples harvested and packed as for export. After storage for four weeks at -0.5°C and one week at l0°C, bunch quality was evaluated according to standard industry norms. As a preliminary determination anthocyanin and phenol absorbance levels were determined during one season to possibly complement visual evaluation of berry colour. The best overall result with regard to the timing of application for both Flame Seedless and Bonheur was obtained with an ethephon application two weeks after acid maximum (30 to 40% colour). Significant seasonal differences were observed regarding various quality variables of Bonheur. This timing was not very critical for improved export pack-out or meeting the berry colour quality norm. Treatments closer to harvest increase the risk of quality defects during cold storage, such as berry crack, S02 burn, as well as soft and loose berries. An ethephon dosage of 300 mg/L for Flame Seedless and 200 mg/L for Bonheur gave satisfactory results regarding export pack-out, export-quality berry colour at harvest and post-cold-storage quality. The preliminary evaluation of anthocyanin and total phenol absorbance levels did not correlate well with visual evaluation of berry colour, showing the difficulty of sensory determinations for quality evaluation.

Spray Deposition and Control of Botrytis cinerea on Grape Leaves and Bunches: Part 1 (Table Grapes)

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.