Thomas A. Zitter

The 2009 late blight pandemic in the eastern United States - causes and results

The tomato late blight pandemic of 2009 made late blight into a household term in much of the eastern United States. Many home gardeners and many organic producers lost most if not all of their tomato crop, and their experiences were reported in the mainstream press. Some CSAs (Community Supported Agriculture) could not provide tomatoes to their members. In response, many questions emerged: How did it happen? What was unusual about this event compared to previous late blight epidemics? What is the current situation in 2012 and what can be done? Its easiest to answer these questions, and to understand the recent epidemics of late blight, if one knows a bit of the history of the disease and the biology of the causal agent, Phytophthora infestans.

Resistance to Benomyl and Thiophanate-methyl in Didymella bryoniae from South Carolina and New York

An initial collection of 7 isolates of Didymella bryoniae were grown on media amended with 0, 1, 3.2, 10, 31.2, or 100 mg benomyl per liter. Four isolates grew at all five concentrations of benomyl, but the other 3 isolates did not grow at concentrations > 1 mg/liter. Colony diameter of the four resistant isolates was reduced by 50% at 33.1 mg benomyl per liter, relative to growth on nonamended medium. Of 394 isolates tested, 182 isolates were resistant to benomyl; 178 of these resistant isolates were from South Carolina, 1 was from New York, and 3 were from Florida. Of 196 isolates grown on medium amended with 100 mg/liter thiophanate-methyl, 95 were sensitive and 101 were resistant. Essentially all isolates that were resistant to benomyl were resistant to thiophanate-methyl. In greenhouse tests, watermelon plants were sprayed with 0, 1.5, 15, 150, or 1,500 mg benomyl per liter and inoculated 1 day later with either a sensitive or a resistant isolate of D. bryoniae. Relative percent leaf area diseased was greater (P≤0.02) for the resistant isolate than for the sensitive isolate at ≥1.5 mg benomyl per liter. The occurrence of pathogenic, benzimidazole-resistant D. bryoniae in the eastern United States may reduce the effectiveness of benzimidazole fungicides for gummy stem blight management.

Internal Transcribed Spacer Regions 1 and 2 and Random Amplified Polymorphic DNA Analysis of Didymella bryoniae and Related Phoma Species Isolated from Cucurbits.

ABSTRACT Didymella bryoniae (anamorph Phoma cucurbitacearum) is the causal agent of gummy stem blight, although other Phoma species are often isolated from cucurbit plants exhibiting symptoms of the disease. The molecular and phylogenetic relationships between D. bryoniae and these Phoma species are unknown. Isolates of D. bryoniae and Phoma obtained from cucurbits grown at various geographical locations in the United States were subjected to random amplified polymorphic DNA (RAPD) analysis and internal transcribed spacer (ITS) sequence analysis (ITS-1 and ITS-2) to determine the molecular and phylogenetic relationships within and between these fungi. Using RAPD fingerprinting, 59 isolates were placed into four phylogenetic groups, designated RAPD group (RG) I, RG II, RG III, and RG IV. D. bryoniae isolates clustered in either RG I (33 isolates), RG II (12 isolates), or RG IV (one isolate), whereas all 13 Phoma isolates clustered to RG III. There was greater than 99% sequence identity in the ITS-1 and ITS-2 regions between isolates in RG I and RG II, whereas isolates in RG III, P. medicaginis ATCC 64481, and P. exigua ATCC 14728 clustered separately. On muskmelon seedlings, a subset of RG I isolates were highly virulent (mean disease severity was 71%), RG II and RG IV isolates were slightly virulent (mean disease severity was 4%), and RG III isolates were nonpathogenic (disease severity was 0% for all isolates). The ITS sequences indicate that RG I and RG II are both D. bryoniae, but RAPD fingerprints and pathogenicity indicate that they represent two different molecular and virulence subgroups.

Management of Virus and Viroid Diseases of Crops in the Tropics

Management of plant virus and viroid diseases is a matter of vital importance and concern to the farmer, horticulturist, forester and gardener. It is well established that the virus and viroid diseases in different crops cause enormous losses in terms of quantity and quality of products (see Chapter-3 of Volume-I) (Sastry 2013b). Because of the serious losses they cause to agriculture and horticulture, virus/viroid diseases have acquired great importance in the realm of plant pathology and call for effective management measures against them. These diseases are not amenable to control by direct methods. Unlike fungicides and bactericides, no commercial viricides have yet been developed. However a number of indirect control measures for different crop plant-virus/viroid pathosystems suitable to varied agro ecosystems have been developed with an aim to avoid the pathogen or to minimize their impact on crop yields (Thresh 2003, 2006). As these diseases routinely cause systemic infections and have very effective methods of transmission, preventing their spread has become a complicated problem. Some diseases like Tobacco mosaic virus (TMV), Potato virus X (PVX), Cucumber green mottle virus (CGMMV), Citrus exocortis viroid (CEVd) and Potato spindle tuber viroid (PSTVd) are highly contagious and affect large number of hosts belonging to several families.

Plant Virus and Viroid Diseases in the Tropics

Around the globe, besides fungal and bacterial diseases, both virus and viroid diseases have acquired greater importance in the realm of plant pathology and call for effective management measures as they are responsible for heavy yield losses and are a matter of vital importance and concern to farmers, horticulturists, gardeners and foresters. Understanding disease epidemiology is of vital importance for formulating viable disease management practices in a given agroecosystem. The development and progress of plant disease epidemics are variable from region to region. Epidemiology is not a static process, but rather a dynamic course that varies with a change in the ecology, host, vector and virus systems.

Comparison of three potato leafroll virus antisera and a single beet Western Yellows virus antiserum for luteovirus detection in potato

Three potato leafroll virus (PLRV) antisera, representing European, British Columbian, and Californian isolates, performed similarly in detection of PLRV in ELISA tests of samples collected in three successive years at the Florida certification test plots and in tests of other samples collected in New York State. Although a range of absorbance values occurred, this was probably due to random variation in virus titers of samples rather than the occurrence of different virus strains or differential serological reactions by the antisera. Beet western yellows virus (BWYV) was detected in potato leafroll samples from nine states and provinces in North America. The BWYV-positive samples represented 40% in 1983 and 62.5% in 1984 of the total number of samples tested. These results confirm previous reports on the widespread occurrence of BWYV in potato with symptoms of leafroll.ResumenTres antisueros del virus del enrollamiento de la hoja de la papa (PLRV), representando aislamientos europeos, de la Columbia Británica, y de California, se comportaron similarmente en la detección del PLRV en pruebas ELISA de muestras recolectadas durante tres años sucesivos en las parcelas para pruebas de certificación en Florida y en pruebas de otras muestras recolectadas en el estado de Nueva York. Si bien se observó un amplio rango de valores de absorbencia, éste probablemente se debió a la variación obtenida al azar en las dosificaciones de los virus en las muestras, más que por la presencia de variantes o de reacciones serológicas diferentes de los antisueros. El virus del amarillamiento occidental de la betarraga (BWYV) fue detectado en muestras del enrollamiento de la papa correspondientes a nueve estados y provincias de los Estados Unidos. Las muestras positivas de BWYV representaron 40% en 1983, y 62,5% en 1984, del número total de muestras probadas. Estos resultados confirman los informes existentes sobre la amplia distribución del BWYV en papas con síntomas de PLRV.

Ecology and Epidemiology of Virus and Viroid Diseases of Tropical Crops

Epidemiology deals with the occurrence of diseases in plant populations over time and space. A plant virus epidemic results from interactions between virus, host plant, vectors and environmental conditions. Every epidemic can be considered to be a unique pathosystem in which each of the components contributes to the epidemics or outbreak of the disease. Epidemiology is not a static process but a dynamic course that varies with a change in the ecology, host, vector and virus systems. Primarily the effect of environmental conditions on virus disease development becomes apparent when influence of disease development is considered in relation to temperature, rainfall, humidity, vector population and so on. The infected sources like weeds and wild hosts, infected seeds and/or adjacent infected crops can also play a major role for high virus incidence. Similarly, crop growing seasons, cropping practices and host genetic vulnerability are also important factors for epidemics to occur. The progress of disease varies with respect to virus, vector, location, and weather conditions. The movement of the vector within a crop field is related to prevailing meteorological conditions. Genetic RNA or DNA recombination is a major factor responsible for the emergence of new viral strains of species. The epidemiology of plant virus diseases is concerned to the cyclical development of diseases within the plant populations in time and space. Aspects like disease gradients, progress curves, spatial dynamics and meta-populations, systems analysis and simulation models are also discussed in detail. The factors that deal with how and why a virus spreads by vectors in an ecosystem are fundamental to the choice and improvement of control measures in agriculture. The authors herein have focused on the latest information pertaining to ecology and epidemiology of major virus and virus-like diseases in the tropics.