Interpretive Summaries

Approaches to Training Practitioners in the Art and Science of Plant Disease Diagnosis. Terry M. Stewart, Institute of Natural Resources, Massey University, Palmerston North, New Zealand; and Victor J. Galea, School of Agronomy and Horticulture, University of Queensland, Gatton, Australia. Plant Dis. DOI: 10.1094/PD-90-0539. Accepted for publication 27 November 2005.

The diagnosis of a plant disease is both an art and a science. It is a skill that is developed with experience and one that is difficult to gain in a traditional plant pathology course. However, technology can assist with this. This paper reports on three approaches to teaching plant disease diagnosis to tertiary-level students. The first approach sees students working computer-based scenarios, similar to the way a player might explore a video game. Examinations can be carried out, tests can be run, and information collected from growers and managers. The students must provide a diagnosis justification and recommendation. The second approach has students constructing “playable” scenarios themselves. In doing so, they are required to think hard about disease etiology and the diagnostic process. Finally, a third approach challenges students with a real-life problem and a real-life client. As they work through their diagnosis, they are required to document their tasks, observations, and reflections using software developed for this purpose.

Status of Cassava Begomoviruses and Their New Natural Hosts in Nigeria. F. O. Ogbe, International Institute of Tropical Agriculture (IITA), Ibadan, Nigeria and National Root Crops Research Institute (NRCRI), Umudike, Umuahia, Nigeria; and A. G. O. Dixon, J. d’A. Hughes, O. J. Alabi, and R. Okechukwu, IITA, Ibadan, Nigeria. Plant Dis. DOI: 10.1094/PD-90-0548. Accepted for publication 21 December 2005.

Cassava mosaic disease (CMD) is the most important virus disease of cassava in sub-Saharan Africa. The disease is caused by several species of cassava begomoviruses. A diagnostic survey was conducted in 2002–03 to determine the status of cassava mosaic begomoviruses in Nigeria and to ascertain whether a virulent Ugandan variant of East African cassava mosaic virus (EACMV-Ug2), one of the species, was present. Of the 418 farms visited, 48% had cassava with moderately severe or severe symptoms, whereas 52% had cassava with mild symptoms. These distributions were at random. Of 1,106 symptom-bearing cassava leaves tested in the laboratory, 74.1% contained African cassava mosaic virus (ACMV) alone and 0.3% had EACMV alone, whereas 24.4% of the leaves samples were mixed infected by the two viruses. No virus was detected in 1.2% of the samples. EACMV-Ug2 and other begomovirus species, such as Indian cassava mosaic virus and South African cassava mosaic virus, were not detected in any of the leaf samples. Most doubly infected plants showed severe symptoms. Two biological variants of ACMV were identified based on symptom expression on cassava in the field. Two new natural weed hosts, Senna occidentalis (L.) Link and Combretum confertum Lams, were identified for ACMV and EACMV. Although the virulent EACMV-Ug2 was not detected, the occurrence of variants of ACMV and the high proportion of mixed infections by ACMV and EACMV leading to severe symptoms could negatively affect cassava production and, therefore, demand appropriate measures to safeguard it in Nigeria.

Factors Affecting the Development of Wood Rot on Lemon Trees Infected With Antrodia sinuosa, Coniophora eremophila, and Nodulisporium sp. M. E. Matheron and M. Porchas, Yuma Agricultural Center, The University of Arizona, Yuma 85364; and D. M. Bigelow, Department of Plant Sciences, Division of Plant Pathology and Microbiology, The University of Arizona, Tucson 85721. Plant Dis. DOI: 10.1094/PD-90-0554. Accepted for publication 29 November 2005.

Brown heartwood rot, which often is found in branches within lemon groves in southwestern Arizona, is caused by two basidiomycete fungi, Antrodia sinuosa and Coniophora eremophila. Another fungus, a species of Nodulisporium, has been recovered from small, dying lemon tree branches with an internal white wood rot. Studies were conducted to compare the extent of wood decay caused by these fungi on (i) lemon tree branches at different times of the year, (ii) different types of citrus, (iii) some desert woody perennial plants, and (iv) lemon tree branches treated with selected fungicides. For all three fungi, the mean length of wood decay columns that developed on inoculated lemon tree branches from May to October was 14.2 cm, whereas the mean length from November through April was 3.8 cm. There was a significant linear correlation between the length of wood decay columns and air temperature for all three pathogens. The mean length of resultant wood decay columns for all time periods in lemon branches inoculated with A. sinuosa, C. eremophila, and the Nodulisporium sp. was 11.8, 5.7, and 9.6 cm, respectively. The severity of wood decay on branches inoculated with A. sinuosa was significantly greater on lemon compared with grapefruit and orange trees. For all three pathogens, wood decay usually developed to a greater extent in inoculated branches of velvet mesquite and salt cedar compared with Mexican palo verde and creosote bush, four common desert perennials found in southwestern Arizona. Compared with nontreated but inoculated lemon trees, the length of wood decay columns in branches inoculated with A. sinuosa, C. eremophila, and the Nodulisporium sp. in the presence of propicanazole was reduced by 78, 94, and 92%, respectively, and, in the presence of azoxystrobin, was suppressed by 70, 80, and 89%, respectively. Current management guidelines focus on minimizing branch fractures and other nonpruning wounds in conjunction with early detection and removal of infected branches before the onset of the increased wood-decay-development period extending from May to October.

Temperature Sensitivity and Efficacy of Wheat streak mosaic virus Resistance Derived from CO960293 Wheat. D. L. Seifers, Professor, and T. J. Martin, Professor, Kansas State University Agricultural Research Center-Hays, Hays 67601; T. L. Harvey, Professor, Department of Entomology, Kansas State University, Manhattan 66506; S. Haber, Cereal Research Center, Agriculture & Agri-Food Canada, Winnipeg, Canada; and S. D. Haley, Soil and Crop Science Department, Colorado State University, Fort Collins 80523. Plant Dis. DOI: 10.1094/PD-90-0623. Accepted for publication 21 December 2005.

This study was conducted to characterize a new source of temperature-sensitive resistance to Wheat streak mosaic virus (WSMV) found in CO960293 wheat. CO960293 wheat was tested for WSMV resistance using 51 WSMV isolates under different temperatures to determine the stability of the resistance and yield trials were conducted in the field for 3 years. No WSMV isolate defeated the resistance of CO960293 at 18ºC. Yield of CO960293 in field trials was reduced in only 1 of 3 years. These findings demonstrate that this wheat line can be a valuable source of resistance to WSMV in wheat programs, particularly in areas where temperatures are cool following planting in the fall.

A Quantitative Assay of Puccinia coronata f. sp. avenae DNA in Avena sativa. E. W. Jackson, J. B. Avant, K. E. Overturf, and J. M. Bonman, USDA ARS Small Grains and Potato Germplasm Research Unit, 1691 S. 2700 W., Aberdeen, ID 83210. Plant Dis. DOI: 10.1094/PD-90-0629. Accepted for publication 26 December 2005.

Crown rust is caused by the fungus Puccinia coronata f. sp. avenae and is the most damaging disease of oat worldwide. To assess crown rust resistance, researchers rely on detailed measurements of symptoms and signs of disease on infected plants. These measurements can be tedious and may be biased by differences among individual raters. To overcome these problems, our objective was to develop a molecular-based assay to estimate fungal growth associated with infected plants by quantifying the amount of fungal DNA present. Using five genotypes with varying levels of resistance, we compared results from the new assay with digital image analysis. We found that estimating fungal presence improved our ability to distinguish various levels of resistance among the genotypes and enabled us to identify resistant genotypes sooner after inoculation. We have developed a new way to measure rust disease resistance and have improved our ability to study the interaction between the crown rust pathogen and oat. Similar assays should prove useful for other cereal rust diseases.

Sugar Beet Performance with Curly Top Is Related to Virus Accumulation and Age at Infection. William M. Wintermantel, USDA-ARS, Salinas, CA; and Stephen R. Kaffka, University of California-Davis. Plant Dis. DOI: 10.1094/PD-90-0657. Accepted for publication 3 January 2006.

Resistance to curly top disease caused by Beet curly top virus (BCTV) and related curtoviruses has been important to sustainable sugar beet (Beta vulgaris) production in the western United States for most of the last century. Recent advances in sugar beet genetics have led to the development of high-yielding cultivars that produce exceptional yields, but have little resistance to curly top disease. These cultivars can produce tremendous yields, but can result in significant losses in years with early infection or abundant curtoviruses. A greenhouse assay has been developed to rapidly test cultivars for a broad array of factors affecting performance in the presence of curly top. Previous studies have shown that sugar beet plants were more susceptible and losses more severe when seedlings were infected by BCTV, but less severe when plants were larger at the time of infection. To evaluate more precisely the relationship between age at infection, disease severity, virus accumulation, and yield loss in modern cultivars that were not bred for curly top resistance, individual sugar beet plants varying in degree of resistance and susceptibility to curly top were inoculated with 20 viruliferous beet leafhoppers (Circulifer tenellus) each when plants had two, four, or six true leaves, and maintained in a greenhouse for 6 weeks. When plants were inoculated at the two-leaf stage, all cultivars became severely stunted with high disease ratings and similar rates of symptom development, regardless of resistance or susceptibility of the cultivar. Plants inoculated at four- and six-leaf stages exhibited increasing separation between resistant and susceptible phenotypes, with highly resistant cultivars performing well with low disease ratings and increased plant weights relative to susceptible cultivars. High-yielding cultivars performed only slightly better than the susceptible control cultivar. Results from greenhouse trials matched those from field trials conducted under heavy curly top pressure. Importantly, low virus concentration was directly correlated with lower disease ratings and higher plant weight, while elevated virus concentrations corresponded to higher disease ratings and lower weights. The greenhouse assay developed for these studies provides a rapid and effective preliminary test for selecting cultivars with improved curly top control, and could lead to more rapid incorporation of resistance into high-yielding sugar beet when coupled with subsequent field evaluation.

Development of a Real-Time, Quantitative PCR for Detection of the Columbia Basin Potato Purple Top Phytoplasma in Plants and Beet Leafhoppers. J. M. Crosslin and G. J. Vandemark, USDA-ARS, Vegetable and Forage Crops Research Unit, Prosser, WA 99350; and J. E. Munyaneza, USDA-ARS, Yakima Agricultural Research Laboratory, Wapato, WA 98951. Plant Dis. DOI: 10.1094/PD-90-0663. Accepted for publication 4 January 2006.

The Columbia Basin potato purple top phytoplasma has been identified as a strain of the beet leafhopper–transmitted virescence agent. This bacteria-like pathogen has caused disease outbreaks in the potato-producing regions of Washington and Oregon in the last several years. Detection and identification of the pathogen has traditionally required a molecular test called nested polymerase chain reaction (PCR). This procedure is quite reliable but requires a great deal of time and does not give information on how much of the pathogen is present in a sample. The research reported in this paper describes for the first time a real-time PCR test for detection of the pathogen in potato plants and the beet leafhoppers responsible for transmission of the phytoplasma. The test is relatively rapid and provides information on how much of the pathogen is present.

Optimizing Soil Disinfestation Procedures for Fresh Market Tomato and Pepper Production. D. O. Chellemi, USDA, ARS, U.S. Horticultural Research Laboratory, 2001 South Rock Rd., Fort Pierce, FL 34945; and J. Mirusso, Mirusso Enterprises, Delray Beach, FL. Plant Dis. DOI: 10.1094/PD-90-0668. Accepted for publication 10 January 2006.

Despite intense efforts to find a suitable replacement, soil fumigation with methyl bromide is still considered critical for the production of fresh market tomato and pepper in the United States. Preliminary studies demonstrated the absence of a “stand alone” replacement chemical that could achieve a spectrum of pest control similar to that of methyl bromide while maintaining a high level of marketable yields. Subsequent efforts identified a mixture of the soil fumigants 1,3-dichloropropene (1,3-D) and chloropicrin (Pic) applied in combination with an herbicide (pebulate) as the best alternative. To ensure that chemical alternatives are used in the most effective, economical, and environmentally sound way possible, they should be integrated into soil disinfestation programs that improve their spectrum of pest control and minimize their negative impact on the environment. The current research was initiated to investigate combinations of 1,3-D and Pic, soil solarization, and virtually impermeable films (VIF) in several application sequences for their effects on soilborne pests and marketable yield of fresh market pepper and tomato. To permit the fumigation of existing plastic mulched beds, a novel apparatus referred to as an “under bed fumigator” was used to inject fumigants into the soil. Application of fumigants under VIF dramatically improved their retention in the soil. Survival of the fungus responsible for causing Fusarium wilt of tomato (Fusarium oxysporum f. sp. lycopersici) in soil was reduced when fumigation or solarization was applied under a VIF compared with untreated controls. Significant control of yellow and purple nutsedge and root-knot nematodes was achieved with reduced fumigant dose using a VIF. Marketable yield of pepper increased when fumigant applications were combined with a VIF or delayed 7 days after planting beds were covered with low-density polyethylene plastic. The results demonstrate that chemical and nonchemical soil disinfestation methods can be combined with novel application equipment and procedures to improve the spectrum of pest control and reduce the effective fumigant application rates. Thus, by optimizing soil disinfestation procedures, effective pest control was achieved using methods that mitigate the potential for environmental contamination or human exposure to soil fumigants.