Since 2004, a new leaf blight disease on garlic of high severity has been observed in Dangyang County, Hubei province, China. Initial symptoms consisted of multiple, small, irregular to oval, white leaf spots, which enlarge to produce sunken purple lesions, sometimes surrounded by a bright yellow margin. As the disease progressed, lesions expanded and merged, resulting in withering of leaf tips. After isolation and pathogenicity testing, the causal agent of leaf blight of garlic was identified as Stemphylium solani from cultural and morphological characteristics, and subsequent analysis of the internal transcribed spacer region of ribosomal DNA. When fungal plugs of two S. solani isolates were inoculated onto 11 garlic cultivars and 20 other crop species, leaf spots appeared on all inoculated plants, but two garlic cultivars (Qingganruanye and Ruanruanye) and three crop species (Capsicum annuum, Brassica napus and Amaranthus mangostanus) showed the smallest leaf spots. In cross-inoculation experiments, no indications of host specificity were observed, but S. solani isolated from garlic was generally the most virulent on five plant species, while the isolate from leek (Allium odorum) was generally the least virulent. Toxicity testing of the crude culture filtrates indicated that garlic isolates produced toxin(s) that were not heat-labile and induced different levels of phytotoxicity toward various garlic cultivars and crops.

Stemphylium solani, the causal agent of leaf blight of garlic (Allium sativum), produced phytotoxic metabolites in culture. A non-host-specific phytotoxin from culture filtrate of S. solani isolate DY-5, named SS-toxin, was extracted by ethyl acetate, isolated by bioassay-guided thin layer chromatography on silica gel, and purified by preparative liquid chromatography. SS-toxin produced necrotic lesions on detached garlic leaves, similar to that caused by S. solani. When wounded leaves were each treated with a 10-μl droplet of toxin, Changbanpo, a garlic cultivar susceptible to leaf blight, showed necrotic lesions at 11 μg/ml toxin, while a resistant cultivar, Qingganruanye, showed symptoms at 44 μg/ml. The effective doses causing 50% inhibition (EC50 values) of root growth and shoot elongation of the susceptible cultivar were 64.9 and 178.5 μg/ml, respectively. The SS-toxin significantly inhibited mitotic activity of root tip cells of the susceptible cultivar from 22 μg/ml and higher causing an abnormally high frequency of cells in interphase. Concentrations over 50 μg/ml of SS-toxin were found to be significantly toxic to total chlorophyll, both chlorophyll a and b, of the susceptible cultivar. The plasma membrane–cell wall interface, nuclear membranes, mitochondria, and chloroplasts were affected by SS-toxin in susceptible leaf cells. This is the first report of the purification and testing of a phytotoxin produced by S. solani.

No abstract

Eleocharis dulcis is a perennial herbaceous plant in the family Cyperaceae, which is native to China and India where it grows well in moist-to-wet soils. It is commonly used as a fruit or a vegetable. From August 2009 to December 2010, symptoms were observed on E. dulcis stems in Tuanfeng County, Hubei, China, with the crop area affected estimated to be more than 1,300 ha per year. Corm yield was reduced by 20% on average with as much as 60% yield losses in some fields. Lesions were initially small, red-brown, and oval or circular that enlarged to produce apical necrosis and extending until the stems withered, usually within 2 months. To obtain isolates, diseased tissue was disinfested for 1 min in 0.1% mercuric chloride solution, rinsed with sterilized water, and plated on potato dextrose agar. Isolates with similar morphological characteristics were consistently recovered. Three isolates, CTF-3, CTF-10, and CTF-11, were used to further evaluate characteristics of the pathogen. After 7 days, white colonies were 76 to 80 mm across on oatmeal agar (OA) with sparse aerial hyphae and a slight salmon color in the conidial masses. Pycnidia produced on OA were globose to subglobose, usually with one slightly ostiolar papilla, olivaceous to olivaceous black, and 93 to 245 μm in diameter. Conidia were hyaline, unicellular, ellipsoidal, mostly with two polar guttules, and 3.6 to 6.2 × 2.0 to 3.3 μm. Chlamydospores were absent. Growth of the isolates on malt extract agar (MEA) was slower than on OA, and the colony diameters at 7 days were 60 to 65 mm. The reactions with 1M NaOH were both positive on OA and MEA where the cultures initially changed to yellow green and gradually turned to red. The pathogen was identified as Phoma bellidis Neerg. based on descriptions in Boerema et al. (2). Pathogenicity tests were performed with the three isolates in the laboratory by spraying conidial suspensions (1 × 106 conidia/ml) containing 0.1% Tween 20 until runoff (30 ml per plant) onto stem surfaces of 50-day-old, 60 cm tall plants. For each isolate, there were 50 stems from five replicate plants that had multiple stems per plant. Control plants were treated with sterilized water containing 0.1% Tween 20 only. Plants were incubated with a 16-h photoperiod at 28°C and 90% relative humidity in an artificial climate chamber. Five days after inoculation, typical red-brown spots were observed on all inoculated stems but no symptoms were seen on water-treated control plants. Koch's postulates were fulfilled by reisolation of P. bellidis from diseased stems. The pathogenicity tests were repeated twice more with the same results. P. bellidis has only been reported previously on Bellis spp. from England, Denmark, Italy, the Netherlands, and Switzerland (1,2). Furthermore, there are only a few fungal diseases known to be associated with E. dulcis, and none so far that involve species of Phoma (3,4). To our knowledge, this is the first report of P. bellidis infecting E. dulcis worldwide. References: (1) M. M. Aveskamp et al. Stud. Mycol. 65:27, 2010. (2) G. H. Boerema et al. Phoma Identification Manual: Differentiation of Specific and Infra-Specific Taxa in Culture. CABI Publishing, Wallingford, UK, 2004. (3) P. L. Lentz. Am. Midl. Nat. 67:184, 1962. (4) L. Pan et al. J. Changjiang Vegetables (in Chinese) 14:10, 2010.

Houttuynia cordata is a perennial herbaceous plant (family Saururaceae) that is native to southern China, Japan, Korea, and Southeast Asia where it grows well in moist to wet soils. It is commonly used as a Chinese herbal medicine and as a vegetable. In North America and Europe it is also used as an ornamental. From September 2007 to November 2009, symptoms of leaf spot were found on H. cordata leaves in Dangyang County, Hubei, China, with the crop area affected estimated to be over 600 ha per year. Rhizome yield was reduced by 20% on average, with up to 70% yield losses in some fields during the autumn growing season. Lesions were initially small, brown, and oval or circular that developed into dark spots and sometimes formed target spots with white centers. These spots enlarged and overlapped, extending until the leaves withered entirely usually within 2 months. A fungus was consistently recovered from symptomatic leaf samples collected in October 2008 or 2009 with an average 90% isolation rate from ~60 leaf pieces that were surface sterilized with 0.1% mercuric chloride solution. Three isolates, HCDY-2, HCDY-3, and HCDY-4, were used to further evaluate characteristics of the pathogen. On potato dextrose agar, all cultures initially developed white colonies and the centers turned gray or brown after 4 days of incubation. Conidiophores were single or fasciculate, straight or knee curved, gray-brown with regular septa, and 42 to 61 × 4 to 5 μm. Conidia were obclavate or ovate, brown, and 26 to 38 × 12 to 20 μm with three to five transverse and one to three longitudinal or oblique septa. The tops of some conidia developed into secondary conidiophores, which were cylindrical, beige, and 5 to 17 × 3 to 5 μm. The pathogen was identified as Alternaria alternata based on descriptions in Simmons (3). Genomic DNA of HCDY-2 was extracted, and the rDNA-internal transcribed spacer sequence showed 99.6% identity to A. alternata (GenBank No. AY513941). Pathogenicity tests were performed with the three isolates by spraying conidial suspensions (1 × 106 conidia/ml) containing 0.1% Tween 20 onto upper and lower surfaces of leaves of 40-day-old 15-cm high plants. There were 20 leaves from five replicate plants for each isolate. Control plants were treated with sterilized water containing 0.1% Tween 20 only. All plants were incubated with a 16-h photoperiod at 25°C and 90% relative humidity in an artificial climate chamber. Five days after inoculation, typical brown spots were observed on all inoculated leaves but no symptoms were seen on water-treated control plants. Koch's postulates were fulfilled by reisolation of A. alternata from diseased leaves. The pathogenicity tests were carried out twice. A survey of the literature revealed only a few fungal diseases associated with H. cordata (1,2,4), including Phyllosticta houttuyniae, Pseudocercospora houttuyniae, Rhizoctonia solani, and Sclerotium rolfsii. Although A. alternata is a cosmopolitan plant pathogen, it has not been reported on any species in the four genera in Saururaceae (Anemopsis, Gymnotheca, Houttuynia, and Saururus) (3). To our knowledge, this is the first report of A. alternata infecting H. cordata worldwide. References: (1) Y. L. Guo and W. X. Zhao. Acta Mycol. Sin. 8:118, 1989. (2) K. Sawada. Spec. Publ. Taiwan Univ. 8:138, 1959. (3) E. G. Simmons. Alternaria: An Identification Manual. The American Phytopathological Society, St. Paul, MN, 2007. (4) Y. Wu et al. J. Changjiang Vegetables (In Chinese) 2:19, 2007.