First Report of Ascochyta spp. on Soybean in South Africa.

During February 2000 soybean fields over a wide area in South Africa were affected by a previously unreported disease. A typical target spot developed on leaves and many became blighted. Elongated dark brown to black lesions developed on the stems, often resulting in wilting of young shoots. Petioles and leaf axils were colonized, resulting in premature leaf drop. The most severe manifestation was on pedicels where infection suppressed podfill and plants remained green as ripening was delayed. Two distinct groups of isolates were obtained from lesions. In both groups an Ascochyta anamorph was present, while from some lesions collected in Kwazulu-Natal and Mpumalanga, a teliomorph (Mycosphaerella) was also present. Since dry beans are produced in these areas and M. phaseolorum is common on this crop, it was believed that the pathogen might have moved to soybean. In cross inoculations, both soybean and dry bean isolates were pathogenic to both hosts. However, M. phaseolorum isolates were more aggressive to both hosts than the Ascochyta sp. Morphologically the anamorphs of two types of isolates were indistinguishable. DNA was isolated from freeze dried mycelia using a modified version of the CTAB-method described by Graham et al. (1). The DNA concentration and purity were estimated by measuring absorbances at A260 and A280. Genetic difference between both isolates were determined using amplified fragment length polymorphism (AFLP) technique. The AFLP analysis was performed following the protocol described by Vos et al. (2) and the product manual supplied by Life Technologies Inc. (Gaithersburg, MD) with minor modifications. Five randomly selected primer pair combinations were tested for their ability to reveal polymorphisms between the isolates. The gel electrophoresis for AFLP products was as described by Vos et al. (2). AFLP gels were silver stained following the protocol described by silver sequence DNA sequencing system manual (Promega, Madison, WI). All five primer pairs revealed only polymorphisms between isolates. No corresponding bands between the two isolates were detected using these five primer pair combinations. It is concluded that both M. phaseolorum and an unidentified Ascochyta sp. were the cause of the epidemic. Ascochyta spp. have not previously been reported on soybean in South Africa. References: (1) G. C. Graham et al. Biotechniques 16:48-50, 1994. (2) P. Vos et al. Nucleic Acids Res. 21:4407-4414, 1995.

Occurrence of Puccinia xanthii on Sunflower in South Africa.

A severe outbreak of a foliar disease occurred on ornamental sunflower (Helianthus annuus L. cv. Tyao) cultivated under plastic protection near Potchefstroom in the North West Province during February 2000. According to previous descriptions of host range, symptoms, and spore morphology, the pathogen was identified as the microcyclic rust fungus, Puccinia xanthii Schwein. (2-4). Telia, reaching 9 mm in diameter and often coalescing to form larger lesions associated with chlorosis, occurred on the lower surface of leaves. Symptoms were first observed at the V-12 growth stage when approximately 40% of leaves displayed lesions. The disease subsequently spread to younger plants (V-2 stage) where an incidence of 100% was recorded. Severity estimates regularly reached 50% leaf area affected. The primary source of inoculum was thought to be nearby rust-infected Xanthium strumarium sensu lato (burweed or cocklebur) plants, a known host for P. xanthii (2,4). Infection studies were carried out on 3-week-old sunflower plants using a mini settling tower supporting an inverted water agar plate containing leaf disks with P. xanthii telia (2) obtained from either sunflower or burweed. Plants covered with a settling tower were kept in the dark in a dew chamber for 48 h. Microscopic examination of inoculum from both sources confirmed germination of teliospores and formation of basidiospores. Test entries had similar disease reactions regardless of whether inocula were collected from H. annuus or X. strumarium. Fleck symptoms were visible on the hybrids SNK 77 and Orit 5 days after placing plants in the dew chamber. On these entries, telia gradually increased in size, indicating susceptibility to P. xanthii. Furthermore, those leaves receiving high inoculum dosages were desiccated 11 days after inoculation. Hybrids Hysun 333 and CRN 1470 and the male sterility maintainer lines DC 4 and RK 74 produced resistant reactions. Hyperplasia of stem and petiole tissue was observed on certain Orit, DC 4, and CRN 1470 plants. Susceptibility of burweed, a common and widespread annual weed in southern Africa (1), to a sunflower isolate of P. xanthii, was also confirmed. Depending on the formation of teliospores, the identity of the causal organism was verified in all inoculation studies. P. xanthii damage on sunflower has not been reported from South Africa. However, the disease has now become economically important as severe foliar lesions rendered plants unacceptable for the ornamental sunflower market. References: (1) C. Bromilow. 1995. Problem Plants of South Africa. Briza Publications, Pretoria, South Africa. (2) L. Morin et al. Can. J. Bot. 71:959, 1993. (3) L. Morin et al. Mycol. Res. 96:661, 1992. (4) J. Parmelee. Can. J. Bot. 47:1391, 1969.

Occurrence of the White Rust Pathogen, Albugo tragopogonis, in Seed of Sunflower.

White rust of sunflower (Helianthus annuus L.), caused by Albugo tragopogonis (Pers.) S. F. Gray, appeared in South African fields not previously planted to sunflower. Spread to these fields from infested fields was unlikely, as some of the newly affected fields were planted out of season and were more than 300 km away from other sunflower production fields. Several reports of this nature led us to investigate the possibility of seed transmission of the causal organism. Extensive colonization of sunflower heads by A. tragopogonis was observed in field trials and breeding nurseries. Head infections consisted of two distinct lesion types. White rust pustules, typical of those formed on abaxial sides of leaves, were recognized on abaxial sides of involucral bracts. Grayish, localized lesions containing dark-colored oospores of the fungus, similar to those formed on stems and petioles (1), were produced in sub-epidermal tissue and extended 3 to 5 mm deep into receptacles. Colonization of seeds was found in only a few lines. Oospores were produced in the pericarps and testae of seeds. No oospores or hyphae, however, were observed in the embryo. This is the first report of A. tragopogonis being seed-borne. Since the incidence of seed infection is low, spread of disease to infested fields is expected to be insignificant. Of more concern, however, is the possible long-range dissemination of the fungus by means of infected seed into regions or countries where the disease has not been previously reported. Reference: (1) P. S. Van Wyk et al. Helia 18:83, 1995.

Net Blotch and Necrotic Warts Caused by Streptomyces scabies on Pods of Peanut (Arachis hypogaea).

Peanut (Arachis hypogaea L.) grown in rotation with potato (Solanum tuberosum L.) in the Northern Cape Province and at one locality in the Free State Province, South Africa, have developed unusual pod symptoms since the 1994 to 1995 season. Symptoms ranged from a net blotch with scattered lesions to dark brown, necrotic, wartlike lesions on the cv. Sellie. Streptomyces scabies (2) was consistently isolated from both types of lesions. Pathogenicity was confirmed in greenhouse tests. Inoculum was prepared by growing colonies on yeast malt extract agar for 21 days. Ten milliliters of sterile, distilled water was poured over the colonies of two different isolates and lightly scraped with a sterile needle. Separate sets of sterile soil were amended with spore suspensions of different isolates at a rate of 10 ml/kg and thoroughly mixed. Seeds of cv. Sellie and minitubers of potato cv. BP1 were planted in infested soil in 3-kg plastic bags. Uninfested soil served as controls. Each set of pots for both peanut and potato had three replications. Pots were kept in a glasshouse at 27°C for 12 weeks. Plants were lifted, disease development recorded, and infected plant parts prepared for reisolation on antibiotic-amended water/ agar according to the protocols of Loria and Davis (3). Both lesion types recorded under field conditions developed on peanut pods in the glasshouse upon reinoculation. Incidence ranged between 2 and 3 pods out of 8 to 10 pods per plant while minitubers were nearly 100% infected. Streptomyces scabies was isolated from lesions on both peanut and potato. Net blotch caused by an unknown Streptomyces sp. is a significant problem on peanut pods in Israel (1). However, the species found in Israel was not identified and could be different from the one reported here, according to Y. Barash (Tel Aviv University, personal communication). This is a first report of S. scabies being pathogenic on peanut in South Africa. References: (1) Y. Barash et al. (In Hebrew.) Hassadeh 72:688, 1992. (2) D. H. Lambert and R. Loria. Int. J. Syst. Bacteriol. 39:387, 1989. (3) R. Loria and J. R. Davis. Streptomyces scabies. Pages 114-119 in: Laboratory Guide for Identification of Plant Pathogenic Bacteria. 2nd ed. N. W. Schaad, ed. American Phytopathological Society, St. Paul. MN, 1988.

Occurrence of Downy Mildew on Sunflower in South Africa.

Downy mildew, caused by Plasmopara halstedii (Farl.) Berl. & De Toni in Sacc., is an economically important disease of sunflower (Helianthus annuus L.) in Europe and the United States (1). The disease was first noticed in South Africa in a commercial field near Standerton and in a seed production field near Kroonstad during the 1993 to 1994 planting season. During the 1995 to 1996 season, downy mildew was found in experimental hybrids near Heilbron, and in commercial fields near Heil-bron, Marikana, and Potchefstroom. At Heilbron, five hybrids were infected with P. halstedii, whereas three others showed symptoms of downy mildew at Potchefstroom and Marikana. All commercially cultivated hybrids have been developed in South Africa. Disease incidence in all the fields was low, with less than 1% of plants affected by the disease. Diseased plants were dwarfed and displayed veinal chlorosis of leaves typically associated with downy mildew of sunflower. White fungal growth commonly occurred on lower leaf surfaces. Sunflower seedlings inoculated with P. halstedii produced symptoms characteristic of downy mildew. The occurrence of the disease in many geographic areas and on various hybrids in South Africa suggests that the fungus is well established. Recent outbreaks can be attributed to the cool, wet, climatic conditions of the 1993 to 1994 and 1995 to 1996 seasons. The susceptibility of local hybrids suggests that downy mildew is a potentially dangerous disease of sunflower in South Africa. Reference: (1) J. F. Miller and T. J. Gulya. Crop Sci. 27:210, 1987.