Plant Disease magazine, November 2004 issue, Volume 88, Number 11
Selection of interpretive summaries of articles relevant to seed professionals

Researchers Discover Cause of and Control Methods for Destructive Bell Pepper Disease

Identification and Management of Colletotrichum acutatum on Immature Bell Peppers.
Melanie L. Lewis Ivey, Cristian Nava-Diaz, and Sally A. Miller, Department of Plant Pathology, The Ohio State University, Ohio Agricultural Research and Development Center, Wooster 44691. Plant Dis. D-2004-0823-01R, 2004 (online). Accepted for publication 10 June 2004.
A severe form of anthracnose attacking both immature and mature pepper fruit appeared in Ohio and other states in the late 1990s. The pathogen causes multiple lesions on the fruit, sporulates quickly and profusely, and spreads rapidly throughout the pepper crop, resulting in up to 100% yield loss. The cause of the disease was determined to be Colletotrichum acutatum based on morphology and results of a polymerase chain reaction (PCR) assay with primers specific for this pathogen. The pathogen also caused lesions on tomato and strawberry fruit. All bell pepper cultivars tested for response to C. acutatum were susceptible in field trials, although they varied in degree of susceptibility. ‘Crusader’, ‘Valiant’, and ‘ACX229’ were the most susceptible, while ‘North Star’ and ‘Paladin’ were least susceptible. The fungicides pyraclostrobin (Cabrio) alternated with manganese ethylenebisdithiocarbamate (Manex), chlorothalonil (Bravo Ultrex) alone, Manex plus copper hydroxide (Kocide 2000), and pyraclostrobin + boscalid (BAS 516 = Pristine) alternated with Manex significantly reduced anthracnose incidence and intensity in bell peppers compared with the untreated control. Of these fungicides, only Cabrio, Manex, and Kocide are currently labeled for use on peppers.

Study Identifies Source of Foliar Diseases that Affect Seed-Producing Lucerne in France

Common and Newly Identified Foliar Diseases of Seed-Producing Lucerne in France
C. Leyronas, UMR Epidémiologie végétale et Ecologie des populations, INA PG-INRA, BP 01, 78850 Thiverval-Grignon, France; L. M. Broucqsault, FNAMS, Ferme de Marcellas, 26800 Etoile, France; and G. Raynal, UMR Epidémiologie végétale et Ecologie des populations INA PG-INRA, France. Plant Dis. D-2004-0913-01R, 2004 (online). Accepted for publication 17 June 2004.
Lucerne grown for seed production is infected by many fungi, forming spots on leaves that may lead to seed yield losses. In France, the leading European producer of lucerne seed, no current data about these pathogens were available and what was available was mostly about fodder lucerne. French lucerne seed growers needed a survey to determine which fungi were present in the three main production regions. This study surveyed the three regions over 4 years and showed that nine fungi were responsible for spots on lucerne leaves. Among them, three fungi previously considered unimportant or never before reported were found at high levels in certain regions and must be taken into account in lucerne culture management.

Scientists Develop Rapid Detection Method for Bacterial Leaf Blight of Carrot

Development of PCR-Based Assays for Detecting Xanthomonas campestris pv. carotae, the Carrot Bacterial Leaf Blight Pathogen, from Different Substrates
X. Q. Meng, K. C. Umesh, R. M. Davis, and R. L. Gilbertson, Department of Plant Pathology, University of California, Davis 95616. Plant Dis. D-2004-0830-01R, 2004 (online). Accepted for publication 18 June 2004.
Commercial carrot production is plagued by a number of diseases caused by bacteria and fungi. Some of these disease agents are carried on the seed, thereby allowing for early infection of plants in the field and effective long-distance spread of the pathogens. One such disease is bacterial leaf blight of carrot, caused by Xanthomonas campestris pv. carotae. This is the most important bacterial disease of carrot, and it can cause yield losses to carrot production under favorable conditions (high temperatures and moisture). Current methods for the detection and identification of this bacterium can take as long as 4 weeks, mostly due to the need to carry out time-consuming and laborious carrot plant inoculation tests to confirm the pathogen’s identity. Furthermore, such tests require access to a greenhouse or growth chamber. In this report, we describe the development and application of a new rapid detection method for X. campestris pv. carotae that is based upon the specific amplification, mediated by polymerase chain reaction (PCR), and detection of a fragment of the bacterial genetic material (DNA). This method was successfully used for the detection of X. campestris pv. carotae colonies on agar plates and from carrot leaf tissues and seed. In contrast to the 4 weeks required for identification by the previous methods, this new PCR test requires 24 to 48 hours to complete, and it is highly specific for X. campestris pv. carotae. The PCR-based seed assay that was developed in this work could greatly reduce the time and cost of conducting routine tests of carrot seed lots for X. campestris pv. carotae. Thus, this new biotechnological tool for rapid and specific detection of this plant pathogenic bacterium has the potential to improve management of bacterial leaf blight of carrot and reduce economic losses caused by this disease.

Biological Control Agents May Be Effective in Suppressing Blossom Blight of Alfalfa

Biological Control of Blossom Blight of Alfalfa Caused by Botrytis cinerea Under Environmentally Controlled and Field Conditions
G. Q. Li, Department of Plant Protection, Huazhong Agricultural University, Wuhan, 430070, China; and H. C. Huang, S. N. Acharya, and R. S. Erickson, Agriculture and Agri-Food Canada, Lethbridge Research Centre, PO Box 3000, Lethbridge, AB, T1J 4B1, Canada. Plant Dis. D-2004-0901-01R, 2004 (online). Accepted for publication 25 June 2004.
Blossom blight of alfalfa caused by Botrytis cinerea is an important disease for alfalfa seed production in western Canada. The pathogen relies on exogenous nutrients from senescent petals and pollen grains in the phyllosphere to initiate spore germination and infection of alfalfa pods and seed. Therefore, use of biocontrol agents (BCAs) to inhibit B. cinerea on alfalfa petals may be an effective strategy for suppression of blossom blight. A study was conducted to prove this hypothesis under controlled and field conditions. Of the nine fungal and three bacterial antagonists tested, four fungal BCAs, including Trichoderma atroviride, Gliocladium catenulatum, and Clonostachys rosea, significantly suppressed sporulation of B. cinerea on alfalfa florets, whereas other antagonists, including fungal strains Coniothyrium minitans, T. harzianum, T. viride, Talaromyces flavus, and bacterial strains Bacillus subtilis and Pseudomonas spp. were less effective. Two BCAs, G. catenulatum and C. rosea, effectively suppressed infection of alfalfa pods and seed by B. cinerea when they were applied to both young and senescent petals of alfalfa in a growth chamber. On the basis of the indoor studies, C. rosea strain GR-8 was selected for field trials conducted over 3 years. Results indicated that GR-8 applied to upper parts of alfalfa plants effectively suppressed pod rot and seed rot of alfalfa caused by B. cinerea and significantly increased alfalfa seed production. These studies suggest that C. rosea has potential as a biocontrol agent for control of Botrytis blossom blight of alfalfa.

Fungi Transmitted through Seeds of Certain Forage Grasses  

Acremonium implicatum, a Seed-Transmitted Endophytic Fungus in Brachiaria Grasses
Huang Dongyi and Segenet Kelemu, Centro Internacional de Agricultura Tropical (CIAT), A.A. 6713, Cali, Colombia. Plant Dis. D-2004-0902-03R, 2004 (online). Accepted for publication 25 June 2004.
Brachiaria species are a genetically diverse group of grasses, mostly of African origin, several of which have become commercially important forage grasses, particularly in tropical America. Endophytic fungi live in the spaces between plant cells in a mutually beneficial relationship with their host plant. In exchange for a home, the fungi give the host strength, vigor, and resistance to pests, diseases, and drought. Endophyte–plant associations are widespread in nature. One such fungus, with the scientific name Acremonium implicatum, can develop an endophytic association with Brachiaria species that is asymptomatic. Because this fungus does not cause any visible symptom on its host, it is not possible to recognize the presence of the fungus in the plant visually. Therefore, we developed a rapid DNA-based method that enabled us to detect the presence of the fungus in this plant. Using this method, we examined whether the fungus can preserve itself associated with its host from generation to generation through seed transmission. In this study, we have shown that the endophytic fungus can in fact be transmitted through seed from one generation to the next. The practical implication of seed transmission of endophytes in Brachiaria species is significant: once associated with the plant, the fungus can perpetuate itself through seed, especially Brachiaria grasses that reproduce asexually through seed, for as long as seed storage conditions do not diminish the survival of the fungus. We may be able to exploit this association and its high seed transmission by using a transgenic A. implicatum as a vehicle for production and delivery of gene products of agronomic interest into the host plant to enhance protective benefits and other traits.

For a full list of contents and access to the articles, visit http://www.apsnet.org/pd/+toc/2004/dno04tp.asp .

Plant Disease is a leading international journal of applied plant pathology, published by The American Phytopathological Society. It publishes original research articles focusing on practical aspects of plant disease diagnosis and control. Each issue also includes a monthly feature article summarizing a significant topic in plant pathology. The Disease Notes section contains timely reports of new diseases, outbreaks, and other pertinent observations.