Interpretive Summaries

Occurrence of Resistance-Breaking Isolates of Rice yellow mottle virus in West and Central Africa. Oumar Traoré, Institut de l’Environnement et de Recherches Agricoles (INERA) 01 BP 476 Ouagadougou 01, Burkina Faso; Agnès Pinel and Eugénie Hébrard, Institut de Recherche pour le Développement, IRD, 64501, 34394 Montpellier cedex 5, France; Mawena Y. Dieudonné Gumedzoé, Ecole Supérieure d’Agronomie, Université de Lomé, BP 1515 Lomé, Togo; Denis Fargette, Institut de Recherche pour le Développement, IRD, 64501, 34394 Montpellier cedex 5, France; Alfred S. Traoré, UFR/SVT, Département de Biochimie-Microbiologie, BP 7021 Ouagadougou, Burkina Faso; and Gnissa Konaté, Institut de l’Environnement et de Recherches Agricoles (INERA) 01 BP 476 Ouagadougou 01, Burkina Faso. Plant Dis. DOI: 10.1094/PD-90-0259. Accepted for publication 10 August 2005.

Rice yellow mottle, caused by Rice yellow mottle virus (RYMV), is a major threat to the cultivation of rice (Oryza sativa) in Africa. The disease is widespread in Africa, with estimated yield losses between 25 and 100%, but has not been reported elsewhere. Control measures against rice yellow mottle are mainly directed to breeding for resistance. Most rice cultivars are susceptible to the virus, but the cultivar Gigante and some African rice (O. glaberrima) cultivars like Tog5681, Tog5672, and Tog5675 are highly resistant to RYMV. To develop varieties with better resistance, the high resistance sources are being crossed with high yield susceptible rice lines. However, it has been reported recently that some RYMV isolates were capable (naturally or after serial inoculations) of breaking down the high resistance observed in Gigante and Tog5681. If such resistance-breaking (RB) isolates were frequent in field conditions, it would undermine the stability of resistance to RYMV. In this study, highly resistant rice cultivars Gigante and Tog5681 were challenged with virus isolates from five countries of the West and Central African Sudano-savannah zone in order to investigate the occurrence of RB isolates. High resistance in both cultivars was overcome by 38.6% of the isolates, and RB isolates could be divided into three main pathogenic groups. Isolates in the first group (17.5%) and second group (16.4%) were able to break down resistance in Gigante only and Tog5681 only, respectively. Resistance in both cultivars was overcome simultaneously by isolates of the third group (4.7%). In each group, some isolates induced symptoms, whereas plant infection by others was symptomless. RB isolates occurred in all five countries, with frequencies between 19 and 57%. The wide geographical distribution and high frequencies of RB isolates represent a potential high risk for the durability of resistance to RYMV in the Sudano-savannah zone. This suggests that in addition to breeding for resistance, other control measures like phytosanitation practices are needed in an integrated management frame of RYMV.

Sodium Silicate Reduces Postharvest Decay on Hami Melons: Induced Resistance and Fungistatic Effects. Y. Bi, Key Laboratory of Photosynthesis and Environmental Molecular Physiology, Institute of Botany, Chinese Academy of Sciences, and Department of Food Science, Gansu Agricultural University; S. P. Tian, Key Laboratory of Photosynthesis and Environmental Molecular Physiology, Institute of Botany, Chinese Academy of Sciences; Y. R. Guo and Y. H. Ge, Department of Food Science, Gansu Agricultural University; G. Z. Qin, Key Laboratory of Photosynthesis and Environmental Molecular Physiology, Institute of Botany, Chinese Academy of Sciences. DOI: 10.1094/PD-90-0279. Accepted for publication 23 September 2005.

Hami melons are susceptible to decay caused by various pathogenic fungi. Application of synthetic chemical fungicides is the primary means to control postharvest diseases of fruit. However, concerns about potential impact on public health and environment, as well as development of pathogens resistant to the fungicides applied, have increased the search for alternative control methods. In this study, sodium silicate (Si) was tested for controlling decay caused by Alternaria alternata, Fusarium semitectum, and Trichothecium roseum on Hami melons. Si treatment significantly inhibited mycelial growth of these three pathogens. Si applied at 100 mM was more effective than lower concentrations for controlling decay incidence and severity of fruit inoculated with A. alternata, F. semitectum, and T. roseum and decay incidence of naturally infected melons. Si at 200 mM was phytotoxic. Treated fruit had good appearance. Si treatment did not affect the firmness or total soluble solids of fruit. The protection of Si was correlated with the activation of two families of defense-related enzymes, peroxidase and chitinase. It appeared that induced resistance was an important mechanism of disease control in Hami melons treated with Si. Considering that Si is the second most abundant atom in the Earth’s crust, it is inexpensive and readily available. It has great commercial potential as a decay control product.

Fieberiella florii (Homoptera: Auchenorrhyncha) as a Vector of “Candidatus Phytoplasma mali”. Rosemarie Tedeschi and Alberto Alma, Di.Va.P.R.A. – Entomologia e Zoologia applicate all’Ambiente “Carlo Vidano”, Facoltà di Agraria, Università di Torino, via Leonardo da Vinci 44, 10095 Grugliasco (TO), Italy. Plant Dis. DOI: 10.1094/PD-90-0284. Accepted for publication 27 September 2005.

The apple proliferation phytoplasma represents one of the most economically important threats to apple trees in central and southern Europe. The psyllids Cacopsylla melanoneura and C. picta are acknowledged as the main vectors of that phytoplasma. These insects colonize apple orchards in winter and spring; then they migrate to alternative hosts. The present work, by means of laboratory transmission trials and molecular analyses, demonstrated that another insect, the leafhopper Fieberiella florii, is able to transmit the apple proliferation phytoplasma. Field surveys with yellow sticky traps allowed us to describe the population dynamics of this insect in apple orchards and in wild vegetation areas. Despite the low density and the low transmission efficiency observed, the role of F. florii should not be underestimated. In fact it colonizes the apple orchards in late spring–summer, a period in which the psyllids are not present on apple trees and when the titer of phytoplasmas in apple trees is higher. These aspects, in addition to the high degree of polyphagy of the leafhopper, open new prospects for the epidemiology of apple proliferation and for the control management of the vectors.

Incidence of Phytophthora Blight and Verticillium Wilt within Chile Pepper Fields in New Mexico. S. Sanogo and J. Carpenter, Department of Entomology, Plant Pathology, and Weed Science, New Mexico State University, Las Cruces 88003-0003. Plant Dis. DOI: 10.1094/PD-90-0291. Accepted for publication 30 September 2005.

A systematic field survey was conducted from 2002 to 2004 to provide an appraisal of diseases with wilt symptoms on chile pepper in New Mexico. Wilted plants were found in all 59 fields surveyed. The microorganisms found to be associated with wilt symptoms on chile pepper were Phytophthora capsici and Verticillium dahliae, which cause Phytophthora blight and Verticillium wilt, respectively. From plants exhibiting vascular discoloration in stems, the only microorganism isolated was V. dahliae. From plants with root rot, P. capsici was the microorganism isolated consistently with the highest frequency. These pathogens recovered from wilted plants caused wilting when inoculated onto chile pepper plants under greenhouse conditions, thereby confirming they were the cause of wilt. Results from this study indicate that Phytophthora blight and Verticillium wilt are well established in chile pepper production fields, and that effective management of plant wilting would need to target both P. capsici and V. dahliae.

Reduction of Rhizoctonia Bare Patch in Wheat with Barley Rotations. W. F. Schillinger, Department of Crop and Soil Sciences, Washington State University, Dryland Research Station, P.O. Box B, Lind 99341; and T. C. Paulitz, Root Disease and Biological Control Unit, United States Department of Agriculture–Agricultural Research Service, Washington State University, Pullman 99164-6430. Plant Dis. DOI: 10.1094/PD-90-0302. Accepted for publication 3 October 2005.

The percent area of patches of wheat plants stunted by Rhizoctonia solani AG8 in a long-term dryland no-till cropping systems study in Washington state was significantly reduced during years 6 through 8 when spring wheat followed a year of barley (2-year rotation) compared with a monoculture of continuous annual spring wheat. This is an unusual phenomenon because barley also is highly susceptible to Rhizoctonia spp., indicating a possible microbial-mediated suppression. In addition to less bare patch area, both soft white and hard white classes of wheat had greater grain yield when grown in rotation with barley. Monoculture hard white wheat was more severely affected by Rhizoctonia spp. than soft white wheat. This is the first documentation of Rhizoctonia bare patch disease suppression with rotation of cereal crops in no-till cropping systems.

Characterization of Phytophthora capsici Associated with Roots of Weeds on Florida Vegetable Farms. Ronald D. French-Monar, University of Florida-IFAS, Plant Pathology Department, Southwest Florida Research & Education Center, 2686 State Road 29 North, Immokalee 34142-9515; Jeffrey B. Jones, University of Florida-IFAS, Plant Pathology Department, Gainesville 32611-0680; and Pamela D. Roberts, University of Florida-IFAS, Plant Pathology Department, Southwest Florida Research & Education Center, 2686 State Road 29 North, Immokalee 34142-9515. Plant Dis. DOI: 10.1094/PD-90-0345. Accepted for publication 29 October 2005.

The oomycete Phytophthora capsici is the plant pathogen responsible for Phytophthora blight, a devastating disease of bell pepper, cucumber, pumpkin, and related crops occurring worldwide. This pathogen caused major losses in Florida vegetable production during the past decade, especially on bell pepper and summer squash. Weeds have been shown to be alternative hosts for diverse groups of plant pathogens, including the oomycetes. In weeds, P. capsici had only been recovered from common purslane (Portulaca oleracea) under field conditions prior to this study. We sampled weeds commonly found in nine Palm Beach County, FL, vegetable farms during August 2001, December 2001, and March 2002. Crown and root samples were plated on a semi-selective medium for the presence of P. capsici. Carolina wild geranium (Geranium carolinianum), American black nightshade (Solanum americanum), and Portulaca oleracea were found to be alternative hosts for P. capsici. In greenhouse pathogenicity studies, P. capsici was reisolated from roots when inoculated onto the roots of the weed species the isolate was obtained from and also from some solanaceous weeds tested. Except for Solanum nigrum, no other weed species exhibited plant mortality or disease symptoms. Isolates of P. capsici recovered from weeds were shown to be pathogenic on bell pepper, resistant to the fungicide mefenoxam, and most (92%) were of one of the two mating types (A1). This is the first report of P. capsici naturally occurring on G. carolinianum and S. americanum in vegetable fields. These alternative hosts may play a role in the persistence of this important pathogen in Florida vegetable fields in the absence of a host crop and when the pathogen did not produce oospores, which are survival structures formed when two mating types interact.

Intraplant Sampling of Grapevines for Pierce’s Disease Diagnosis. Rayda K. Krell, Thomas M. Perring, Charles A. Farrar, and Yong-Lak Park, Department of Entomology, University of California, Riverside 92521; and Carmen Gispert, University of California Cooperative Extension, Indio 92201. Plant Dis. DOI: 10.1094/PD-90-0351. Accepted for publication 25 October 2005.

The bacterium Xylella fastidiosa Wells et al. induces Pierce’s disease (PD) of grapevine, which can result in plants dying within just 2 years after infection. There are no treatments that can cure an infected plant. An important component of PD management is to identify and remove infected grapevines. Currently, there are no standard protocols for sampling grapevines within vineyards. This study was initiated to evaluate the efficacy of using typical PD symptoms to identify infected grapevines and determine the best location on a grapevine from which to take plant tissue for diagnostic tests. Several PD symptoms commonly attributed to PD were not reliable indicators of actual PD infection, including leaf necrosis and chlorosis, internodal distance, petiole length and weight, and extent of cane branching. However, the matchstick symptom (abscised leaf blades leaving behind a dried, burnt-appearing petiole tip) was a consistent indicator of infection. Leaves selected from the most basal portions of infected vines had the highest probability of X. fastidiosa detection using an immunological assay. Results from this study suggest that PD symptoms other than matchsticks should not be used to diagnose grapevines and tests for X. fastidiosa in grapevines should use basal plant tissue samples to insure the highest probability of correctly diagnosing grapevines with PD.