S. Denman

Characterisation and pathogenicity of Rhizoctonia isolates associated with black root rot of strawberries in the Western Cape Province, South Africa

Black root rot is an important disease of strawberry caused by a complex of fungi including species of Rhizoctonia. In this study, the Rhizoctonia species and anastomosis groups isolated from diseased strawberries in the Western Cape Province of South Africa were determined and their pathogenicity and relative virulence assessed. Both binucleate and multinucleate types were recovered from diseased roots and identified as R. fragariae and R. solani, respectively. Anastomosis grouping of the isolates was carried out on a sub-sample using the conventional method of hyphal fusion, and molecular techniques were employed to confirm results of the former. RFLP analysis of the 28S RNA gene was used to further characterise relationships among the isolates of Rhizoctonia. The molecular results correlated with those obtained from the conventional methods. In the sub-sample tested, all isolates of R. solani were members of Anastomosis Group 6, whereas three AG types were identified among isolates of R. fragariae, viz. AG-A, AG-G and AG-I at a relative occurrence of 69%, 25% and 6%, respectively. Pathogenicity trials were conducted on 8-week-old cv. Tiobelle plants. All Rhizoctonia isolates tested were pathogenic to strawberry, but R. solani (AG 6) was the most virulent causing severe stunting of plants. R. fragariae AG-A and AG-G were not as virulent as R. solani but also caused stunting. R. fragariae AG-I was the least virulent, and did not cause stunting of the plants; however, it incited small, pale, spreading lesions on infected roots. This is the first species confirmation and AG type identification of Rhizoctonia taxa causing root rot of strawberries in South Africa.

Fungi occurring on Proteaceae. I.

The present study has led to the description of several new fungi occurring on leaves of Protea L., Leucospermum R.Br., Telopea R.Br. and Brabejum L. collected from South Africa, Australia or New Zealand, Cladophialophora proteae L. Viljoen & Crous, Coniothyrium nitidae Crous & S. Denman, Coniothyrium proteae Crous & S. Denman, Coniothyrium leucospermi Crous & S. Denman, Harknessia leucospermi Crous & L. Vilioen, and Septoria protearum L. Viljoen & Crous spp. nov, are described from Protea and Leucospermum in South Africa, while Phyllosticta owaniana G. Winter is redescribed from leaves of Brabejum stellatifolium L. Furthermore, Mycosphaerella telopeae M. Palm & Crous sp. nov. is described from leaves of Telopea collected in New Zealand, while Phyllosticta telopeae H.Y. Yip, which also occurs on this host, is described in culture from Australian material.

Evaluation of fungicides for the control of Botryosphaeria protearum on Protea magnifica in the Western Cape Province of South Africa

A range of fungicides was tested in vitro for their effect on mycelial inhibition. Selected products showing potential for disease control were then further tested under field conditions. The most effective fungicides in the in vitro tests were tebuconazole, benomyl, prochloraz mc, iprodione and fenarimol. In field trials, a 25–85% reduction in the occurrence of stem cankers caused by Botryosphaeria protearum was achieved if fungicides were applied or sanitation pruning was implemented. The best control was obtained with treatments of prochloraz mc alternated with mancozeb. Applications of bitertanol and fenarimol also significantly reduced the occurrence of cankers.

Pathogenicity of Pythium irregulare, P. sylvaticum and P. ultimum var. ultimum to lucerne (Medicago sativa)

The ability of Pythium irregulare, P. sylvaticum and P. ultimum var. ultimum to induce damping-off of lucerne (Medicago sativa) seedlings cv. SA Standard was tested in artificially infested planting medium. Three levels of sand-bran inoculum were used: 5, 25 and 50 g inoculum per kg planting medium. All the Pythium species caused more than 80% damping-off at each inoculum level and are therefore pathogenic to lucerne. There were differences in virulence among the species and isolates tested. Pythium sylvaticum was the most virulent while P. irregulare was the least virulent species tested. The highest survival rates in inoculated soil were recorded for seedlings growing in soil containing the lowest levels of inoculum for each species tested. At the high inoculum levels (25 and 50 gkg) in some cases (P, sylvaticum and P. ultimum var. ultimum) survival was so low that intra-species differences in virulence could no longer be detected. Since there were differences in virulence among the species and isolates tested, it is important to test a number of isolates of a species before drawing conclusions about the pathogenicity of the species as a whole to a particular host, and to select appropriate levels of inoculum to elucidate these differences. As far as could be determined this is the first valid report of P. sylvaticum as a pathogen of lucerne seedlings.

Pathogenicity of Colletotrichum species to Protea cultivars

Colletotrichum species cause a wide range of diseases on Proteaceae. Four Colletotrichum species, C. acutatum, C. acutatum f. sp. hakea, C. boninense and C. gloeosporioides, have been associated with diseased Proteaceae in South Africa. In this study, the pathogenicity of these taxa was evaluated on three Protea cultivars. The relative aggressiveness of the isolates and the effect that wounding had on the host response were compared. Results showed that C. boninense and C. acutatum f. sp. hakea did not cause lesions significantly different from those on the controls. Isolates of C. acutatum and C. gloeosporioides originating from Protea were the primary pathogens associated with Colletotrichum leaf necrosis. Furthermore, C. acutatum was the main cause of anthracnose and stem necrosis of the selected Protea cultivars tested.

Characterisation and pathogenicity of Cylindrocladiella spp. associated with root and cutting rot symptoms of grapevines in nurseries

Species of Cylindrocladiella occur on a variety of hosts, where they are known to act as pathogens or saprobes. Eight species from this genus are currently recognised, of which five have been reported previously in South Africa: C. camelliae, C. elegans, C. lageniformis, C. parva and C. peruviana. Isolates of Cylindrocladiella were obtained from a newly established 99-Richter grapevine mother vine block exhibiting decline symptoms. The DNA phylogeny of these and additional isolates, also obtained from declining grapevines, was determined by sequencing the ITS (ITS 1, ITS2 and 5.8S) as well as (β-tubulin and histone H3 gene regions. This identified four species of Cylindrocladiella on grapevines in South Africa: C. lageniformis, C. parva, C. peruviana and a new species described here as C. viticola, which forms part of the C. infestans species complex. Pathogenicity trials, using stem inoculations on green and 1-year-old 99-Richter cuttings to determine the pathogenicity of selected isolates of these species, gave inconclusive results.

Fusarium wilt: A new disease of cultivated Protea in Southern Africa

A newly recorded disease of cultivated Protea, Fusarium wilt, is described and shown to be caused by Fusarium oxysporum. The disease occurs on mature plants (2-year-old) of P aristata x repens cv. Venus, P cornpacta x susannae cv. Pink Ice, P. cynaroides, P. eximia x susannae cv. Cardinal, P. eximia x susannae cv. Sylvia, P. magnijka x susannae cv. Susara and P. repens cv. Sneyd in the summer rainfall areas of the North-Western province of South Africa and in Zimbabwe. Disease symptoms first become visible as necrotic leaves. Subsequently, a dark lesion develops from the roots along the stem, usually visible only on one side of the stem. Occasionally the lesion develops in the upper part of the stem. The vascular tissue is discoloured leading to branch die-back and plant death. F. oxysporum was readily isolated from the roots, crown and vascular tissues of infected plants. Koch’s postulates were proved on six Protea cultivars. Disease symptoms similar to those observed in the field developed 6 weeks after inoculation on all cultivars. The fungus was re-isolated from the roots; crown and vascular tissues of inoculated plants. This is the first record of Fusarium wilt on Protea plants.

Evaluation of a stem inoculation technique for assessing resistance to Phytophthora cinnamomi in Leucospermum cultivars

An important aspect of integrated disease management of Phytophthora root rot, the most serious root disease of Proteaceae in South Africa, is the use of resistant rootstocks. Most commercial cultivars are propagated from stem cuttings that have been rooted in a rooting bed. Inherent resistance properties should, therefore, be present in the stems. The stem inoculation technique for evaluation of resistance properties of various genotypes is desirable because it is very efficient in terms of space, time, labour and cost. Ten isolates of P. cinnamomi Rands representing the A1 and A2 mating types and a range of aggressive abilities were selected for stem inoculations. The effects on lesion length of depth of inoculation and position of measuring the lesion were assessed. The stem inoculation technique was standardised by using superficial inoculations and measuring the lesion just below the bark after 6 days incubation at 22°C. Lesion development on five Leucospermum cultivars was compared after winter and summer inoculations were carried out over three consecutive years. Results showed that it is important to standardise the stem inoculation technique to reduce variation as a consequence of method. The performance of some cultivars was very consistent over all test periods, but others demonstrated variability in lesion length. Assessment of resistance on results of a single test is thus not recommended.

Fusarium wilt of Agathosma betulina newly reported in South Africa

Fusarium wilt of Agathosma betulina (buchu), caused by Fusarium oxysporum, is newly recorded in South Africa. Koch’s postulates were fulfilled during pathogenicity tests. Further studies are underway to determine the forma specialis of this pathogen.

Chemical control of Phytophthora cinnamomi in potted Leucospermum plants

The efficacy of fosetyl-Al (soil drench), metalaxyl-M (soil drench), fluazinam (soil drench) and potassium phosphonate (foliar spray) in controlling Phytophthora cinnamomi root infections of Leucospermum cvv. High Gold, Sunrise and Spider was compared. The Leucospermum plants were potted, placed in a fibreglass tunnel and inoculated. Fungicide treatments began 4 weeks after inoculation. Stem lesion length was measured and the degree of leaf chlorosis as well as root rot were rated at the conclusion of the experiment. The pathogen was re-isolated from the root and crown of each plant, and soil baiting and dilutions were done to monitor soil populations of P cinnamomi. During the 1997 season, metalaxyl-M gave the best control compared with the other treatments. The combination of metalaxyl-M with cv. Spider completely suppressed symptom development. However, during the 1998 season when climatic conditions were favourable for disease development, no differences were observed between the treatments and the control. Cv. Spider had higher tolerance against the fungus even with the most aggressive isolate and during the warmer 1998 season, indicating the importance of combining chemical control with plant resistance. None of the treatments showed any phytotoxicity.