First Report of White Rust of Rocket (Eruca sativa) Caused by Albugo candida in South Africa

Garden rocket (Eruca sativa syn.: E. vesicaria subsp. sativa (Mill) Thell.) is an annual plant of the Brassicaceae grown for fresh consumption as a salad vegetable. During winter (May to July) of 2013 and 2014 in South Africa, typical symptoms of white rust were observed in two commercial crops (each ~0.5 ha) of the garden rocket cv. Rucola coltivata in Centurion, Gauteng Province, at 33 and 80% incidence, respectively. Symptomatic leaves were deposited in the National Collection of Fungi, Plant Protection Research Institute, Agricultural Research Council, Pretoria, South Africa (PREM 61073). Early infections appeared as white to cream, blister-like sori on the lower leaf surfaces, and pale yellow lesions on the corresponding upper leaf surfaces. Later stages of infection were characterized by coalescing of lesions into large, irregular, necrotic blotches and development of additional sori on the petioles and stems. Sporangiophores were hyaline, clavate or cylindrical, and measured 24 to 30 × 11 to 14 μm (n = 50). Sporangia developed in basipetal chains and were hyaline, globose or polyangular, and 15 to 20 μm (n = 100). Based on these morphological characters and the host plant, the pathogen was identified as Albugo candida (Pers.) Kunze (2). Genomic DNA was extracted using the DNeasy Plant Mini DNA extraction kit (Qiagen) from sori containing sporangia collected from naturally infected leaves, according to the manufacturer's specifications. The internal transcribed spacer (ITS) region of ribosomonal DNA (rDNA) and the cytochrome c oxidase subunit II (COX2) region were amplified and sequenced (1). The ITS (GenBank Accession No. KM588081) and COX2 (KM588082) sequences confirmed identity of the pathogen as A. candida with 100% homology to the corresponding sequences of several A. candida isolates, including DQ418503 for the ITS sequence and DQ418514 for the COX2 sequence, of a voucher specimen of A. candida on E. sativa (BPI 184870) from Pakistan. Inoculum was prepared by scraping sporangia from infected leaves of the cv. Rucola coltivata collected from the 2014 field and placing the material in sterilized, distilled water (SDW) for 12 h at 5°C to induce zoospore formation. Pathogenicity tests were performed by spraying a suspension of 1 × 10⁵ sporangia/ml onto each of 10 5-week-old rocket seedlings of the cv. Rucola coltivata. Ten additional seedlings were inoculated similarly with SDW to serve as a control treatment. The plants were maintained at 12 to 15°C and 95% RH for 72 h (3) before being moved to a shaded greenhouse at 20 to 24°C and 90% RH. Control plants remained symptomless, whereas white rust symptoms similar to those observed in the original fields developed on leaves of inoculated seedlings 10 to 14 days later, demonstrating that A. candida was the causal agent of the disease on E. sativa. To our knowledge, this is the first report of A. candida infecting garden rocket in South Africa. References: (1) Y.-J. Choi et al. Mol. Phylogenet. Evol. 40:400, 2006. (2) K. Mukerji. Descriptions of Pathogenic Fungi and Bacteria No. 458. CMI, Kew, Surrey, UK, 1975. (3) M. J. Sullivan et al. Plant Dis. 86:753, 2002.

Molecular biodiversity of cassava begomoviruses in Tanzania: evolution of cassava geminiviruses in Africa and evidence for East Africa being a center of diversity of cassava geminiviruses

Cassava is infected by numerous geminiviruses in Africa and India that cause devastating losses to poor farmers. We here describe the molecular diversity of seven representative cassava mosaic geminiviruses (CMGs) infecting cassava from multiple locations in Tanzania. We report for the first time the presence of two isolates in East Africa: (EACMCV-[TZ1] and EACMCV-[TZ7]) of the species East African cassava mosaic Cameroon virus, originally described in West Africa. The complete nucleotide sequence of EACMCV-[TZ1] DNA-A and DNA-B components shared a high overall sequence identity to EACMCV-[CM] components (92% and 84%). The EACMCV-[TZ1] and -[TZ7] genomic components have recombinations in the same genome regions reported in EACMCV-[CM], but they also have additional recombinations in both components. Evidence from sequence analysis suggests that the two strains have the same ancient origin and are not recent introductions. EACMCV-[TZ1] occurred widely in the southern part of the country. Four other CMG isolates were identified: two were close to the EACMV-Kenya strain (named EACMV-[KE/TZT] and EACMV-[KE/TZM] with 96% sequence identity); one isolate, TZ10, had 98% homology to EACMV-UG2Svr and was named EACMV-UG2 [TZ10]; and finally one isolate was 95% identical to EACMV-[TZ] and named EACMV-[TZ/YV]. One isolate of African cassava mosaic virus with 97% sequence identity with other isolates of ACMV was named ACMV-[TZ]. It represents the first ACMV isolate from Tanzania to be sequenced. The molecular variability of CMGs was also evaluated using partial B component nucleotide sequences of 13 EACMV isolates from Tanzania. Using the sequences of all CMGs currently available, we have shown the presence of a number of putative recombination fragments that are more prominent in all components of EACMV than in ACMV. This new knowledge about the molecular CMG diversity in East Africa, and in Tanzania in particular, has led us to hypothesize about the probable importance of this part of Africa as a source of diversity and evolutionary change both during the early stages of the relationship between CMGs and cassava and in more recent times. The existence of multiple CMG isolates with high DNA genome diversity in Tanzania and the molecular forces behind this diversity pose a threat to cassava production throughout the African continent.

First Report of Pre- and Postemergence Damping-off of Cowpea Caused by Pythium ultimum in South Africa

During a disease survey of cowpea (Vigna unguiculata (L.) Walp.) in Southern Africa in 1999, a major problem recorded by small-holder rural farmers in areas with wet soil types was reduced seedling emergence. Nongerminated, diseased seeds in these fields were either a brown blotchy color or had a soft rot and disintegrated upon touch. Germinated seedlings failing to emerge above the soil line were characterized by water-soaked lesions girdling the hypocotyl. Emerged seedlings had necrotic taproots and few lateral roots. Infected hypocotyls above the soil line had light brown lesions, and seedlings showed symptoms of wilting. Diseased seeds and seedlings were collected, surface-sterilized, plated on 2% water agar, and Pythium ultimum was isolated. Identification was based on characteristics given in van der Plaats-Niterink (1). Sterile soilbased compost, infested by mixing with P. ultimum mycelia (10 CFU/g of compost), was used to fill 104-cell plastic seedling trays (40 × 28 × 3.5 cm). A single cowpea seed was planted at a depth of 2 cm in each cell, and trays were maintained in a greenhouse at approximately 20°C. Seeds and seedlings showed symptoms identical to those in the field 10 days after planting, and the pathogen was successfully reisolated. Seedlings in noninoculated compost remained symptomless. This is the first report of P. ultimum on cowpea in South Africa. Reference: (1) A. J. van der Plaats-Niterink. Monograph of the Genus Pythium. Centraalbureau voor Schimmelcultures, Baarn, 1981.

First Report of Alternaria cassiae on Cowpea

A destructive foliar disease was observed during field surveys of cowpea (Vigna unguiculata (L.) Walp.) in the Mpumalanga and Gauteng provinces in South Africa. Foliar symptoms begin as semicircular water-soaked lesions at the leaf margins. Lesions enlarge toward the center of the leaf, becoming necrotic. Sporulation is visible with the naked eye on the leaf surface as a black velvet mass. Occasionally, circular lesions are observed in the center of the leaf. Lesions begin as small brown spots, surrounded by a yellow halo. The lesions enlarge and become water-soaked, and black masses of conidia are visible on the brown, necrotic tissue surface. Alternaria cassiae (A. M. M. Juriar & A. Khan) was consistently isolated from the diseased leaf material. An isolate of the causal pathogen was identified by and deposited with the National Collection of Fungi, Plant Protection Research Institute, Pretoria, South Africa, and designated with the number PPRI 6393. Koch's postulates were proven by inoculating the leaves of cowpea seedlings with a 10⁶ conidia ml^-1suspension. Inoculated plants were maintained in a humidity chamber for 48 h and then returned to the greenhouse. After approximately 7 days, symptoms resembling those observed in the field were apparent. A. cassiae was reisolated from diseased tissue. The same foliar disease was observed on cowpea in Botswana. A. cassiae was isolated and identified from the characteristic lesions.

Colletotrichum dematium: Causal Agent of a New Cowpea Stem Disease in South Africa

During surveys of cowpea (Vigna unguiculata (L.) Walp.) fields in the Gauteng province, a destructive stem disease was observed. Stem symptoms began as tan brown discolorations that later spread, becoming dark purplish-brown. Sunken, necrotic lesions and small, black acervuli also were visible on the stems. Colletotrichum dematium (Pers.) Grove was consistently isolated from diseased material. Pathogenicity of an isolate of C. dematium from cowpea stems (deposited with the National Collection of Fungi, South Africa, designated PPRI 6121) was confirmed by inoculating stems of 20 cowpea seedlings with a 10⁵ conidia per ml suspension. Inoculated plants were maintained in humidity chambers for 48 h and then returned to the greenhouse. After 7 days, symptoms resembling those observed in the field were apparent. C. dematium was reisolated from these plants and produced cultures identical to those of the original isolate. This is the first report of C. dematium on cowpea in South Africa.