Interpretive Summaries

Pea Tissue Necrosis Induced by Cucumber Mosaic Virus Alone or Together with Watermelon Mosaic virus. F. Fukumoto, National Agricultural Research Center, Department of Rice Research, Joetu, Inada, Niigata 943-0193 Japan; Y. Masuda, Wakayama Research Center of Agriculture, Forestry and Fisheries, Agricultural Experiment Station, Takao, Kishigawa, Naga, Wakayama 640-0423 Japan; and K. Hanada, National Agricultural Research Center for Kyushu Okinawa Region, Nishigoshi, Kikuchi, Kumamoto 861-1192 Japan. Plant Dis. D-2003-0127-01R, 2003 (online). Accepted for publication 28 October 2002.

Necrotic diseases of the stems, petioles, and leaves of pea plants (Pisum sativum L.), leading to wilting and death, occurred in the Wakayama and Mie Prefectures of Japan. In Japan, Cucumber mosaic virus (CMV) and Pea stem necrosis virus (PSNV) were reported as the causal agents of stem necrosis diseases of pea plant. Based on host range, symptomatology, electron microscopy, and serological relationships, Watermelon mosaic virus (WMV) and three CMV isolates (PE2, PE3A, and PB1), were isolated from diseased plants in the Wakayama Prefecture. In the Mie Prefecture, CMV (PEAN) also was isolated from pea plants with similar symptoms. Single infection with CMV (PB1 or PEAN) caused stem necrosis and eventual death of pea plants. Similar symptoms developed after double infection with WMV and either PE2 or PE3A, whereas single infection with PE2 and PE3A induced symptomless infection in pea plants. We concluded that pea plant stem necrosis was induced by CMV alone or synergistic effects of mixed infection with CMV and WMV. In the pea fields in the Wakayama prefecture, it appeared that necrotic diseases could be caused by single infection with CMV alone or by double infection with CMV and WMV, which viruses could be spread within the pea fields by aphids.

Incidence of Viruses and Viruslike Diseases of Sweetpotato in Uganda. S. B. Mukasa, Department of Crop Science, Faculty of Agriculture, Makerere University, P.O. Box 7062, Kampala, Uganda, and Department of Plant Biology, Genetics Centre, Swedish University of Agricultural Sciences (SLU), Box 7080, SE-750 07 Uppsala, Sweden; Patrick R. Rubaihayo, Department of Crop Science, Faculty of Agriculture, Makerere University, P.O. Box 7062, Kampala, Uganda; and Jari P. T. Valkonen, Department of Plant Biology, Genetics Centre, Swedish University of Agricultural Sciences (SLU), Box 7080, SE-750 07 Uppsala, Sweden, and Department of Applied Biology, University of Helsinki, Finland. Plant Dis. D-2003-0212-02R, 2003 (online). Accepted for publication 7 November 2002.

Sweetpotato is the third most important root crop after potato and cassava. In East Africa, sweetpotato is very important as a starch staple and for food security. However, production is greatly constrained by viral diseases that cause heavy yield losses. Viruslike diseases and viruses were surveyed in 140 fields in 14 districts in the four major agroecological zones of Uganda. Virus disease incidence was low in the short grassland-savannah zone but was high in the tall grass-forest mosaic zone, where up to 20% of the plants in the field displayed symptoms. Sweet potato chlorotic stunt virus (SPCSV) and Sweet potato feathery mottle virus (SPFMV) were detected at a high incidence in all districts. Sweet potato mild mottle virus (SPMMV) and sweet potato chlorotic fleck virus (SPCFV) were detected in 13 and 8 districts, respectively. SPCSV and SPFMV, SPFMV and SPMMV, and SPFMV and SPCFV frequently occurred together in the diseased plants. Co-infections of SPCSV with SPFMV and/or SPMMV were associated with more severe and persistent symptoms than infections with each of the viruses alone. Several plants (11%) displaying viruslike symptoms did not react with the virus antisera used, suggesting more viruses or viruslike agents to be infecting sweetpotatoes in Uganda. The data will contribute to enhanced integrated disease management in sweetpotato crops.

Appressorium Formation and Tomato Fruit Infection by Colletotrichum coccodes. S. Sanogo, Former Graduate Research Assistant, R. E. Stevenson, Senior Research Assistant, and S. P. Pennypacker, Emeritus Professor, Department of Plant Pathology, The Pennsylvania State University, University Park 16802. Plant Dis. D-2003-0203-01R, 2003 (online). Accepted for publication 22 October 2002.

Anthracnose is a disease manifested by fruit rotting in tomato grown for processing, and often reduces marketable yields. The disease is caused by Colleto­trichum coccodes, a soilborne fungus which primarily is disseminated onto tomato foliage and fruit by rain splash. C. coccodes produces structures known as appressoria, which facilitate attachment of the fungus onto plant parts and penetration of plant tissue leading to infection. This study showed that appressoria are rapidly formed at temperatures between 16 and 28°C, and that fruit infection optimally is favored by tem­peratures between 20 and 30°C. Control of tomato anthracnose relies heavily on the use of fungicides. It is suggested that results from this study may be used in designing schemes for applying fungicides when environmental conditions are most favorable for appressoria formation and fruit infection.

Viruses Infecting Cucurbits in Samsun, Turkey. M. A. Sevik and M. Arli-Sokmen, Ondokuz Mayis Universitesi, Ziraat Fakultesi, Bitki Koruma Bolumu, 55139 Samsun, Turkey. Plant Dis. D-2003-0130-01R, 2003 (online). Accepted for publication 2 October 2002.

Samsun Province is one of the main vegetable-growing areas of Turkey and is located in the northern part of the country. Virus diseases cause serious crop losses in cucumber, melon, squash, and watermelon production fields in the region. The major symptoms in infected plants are leaf mosaics and distortions and abnormal fruit color and shape. To detect cucurbit virus diseases and determine their occurrence, surveys were conducted in 18 villages in five districts. Cucurbit plants were found to be infected with Watermelon mosaic virus (WMV), Zucchini yellow mosaic virus (ZYMV), and Cucumber mosaic virus (CMV). WMV was found to be most prevalent virus and detected in 53.9% of the samples tested. ZYMV and CMV infections were determined in 38.8 and 20.6% of cucurbit samples, respectively. WMV, ZYMV, and CMV were detected in 33, 30, and 12 of 45 fields surveyed, respectively. More severe leaf and fruit symptoms occurred because of triple infections of CMV, WMV, and ZYMV determined in 7.9% of the samples. Many different species of vegetables are grown close to each other in the region and growers are not aware of how viruses spread from plant to plant and field to field. Other crops and weeds around cucurbit fields may host these three viruses. The percentages of virus-infected cucurbit plants collected in five districts ranged from 41.6 to 88.1%. All viruses detected in this study are spread by aphids and mechanically. Consequently, there is a high risk of a severe outbreaks in cucurbit fields in the absence of any control measures in the region.

Induced Resistance as a Possible Means to Control Diseases of Strawberry Caused by Phytophthora spp. H. Eikemo, A. Stensvand, and A. M. Tronsmo, The Norwegian Crop Research Institute, Plant Protection Center, Høgskoleveien 7, N-1432 Ås, Norway. Plant Dis. D-2003-0123-01R, 2003 (online). Accepted for publication 4 October 2002.

Crown rot (Phytophthora cactorum) and red stele (P. fragariae var. fragariae) of strawberry are diseases that are difficult to control, partly because the number of available chemicals is limited. Two inducers of disease resistance (acibenzolar-S-methyl and chitosan) were tested for their possible effect on crown rot and red stele of strawberry. Both compounds were effective against crown rot in two strawberry cultivars, and the effect was still evident 20 days after treatment. For red stele, only acibenzolar-S-methyl was effective, and it was still effective when plants were inoculated 40 days after treatment. For both diseases, the effect of the elicitors was equal to the effect of the standard chemical Aliette (fosetyl-Al). In vitro tests showed that the reduction in disease severity in the greenhouse experiments could not be explained by a direct effect of the elicitors on P. cactorum or P. fragariae var. fragariae. These compounds showed a promising effect, and might be future alternatives to chemical control of Phytophthora diseases of strawberry.

Peach Rusty Spot Epidemics: Temporal Analysis and Relationship to Fruit Growth. Laura A. Furman and Norman Lalancette, Rutgers University, Agricultural Research and Extension Center, Bridgeton, NJ 08302-5919; and James F. White, Jr., Rutgers University, Cook College, Department of Plant Biology and Pathology, New Brunswick, NJ 08901-8520. Plant Dis. D-2003-0203-02R, 2003 (online). Accepted for publication 4 November 2002.

Rusty spot of peach, reportedly caused by the powdery mildew fungus Podosphaera leucotricha, is found throughout the United States and can cause significant yield loss on susceptible cultivars. Infection of fruit results in the development of orange-to-brown lesions; death of the epidermal cells causes the surface to become russeted, rendering the fruit unmarketable. Although the spatial aspects of rusty spot epidemics have been studied, little information is available on disease development over time and its relationship to fruit growth. Such information is critical for proper timing of disease control measures. To study these relationships, rusty spot epidemics were monitored throughout the growing seasons of 1999 to 2001. Disease was observed to increase from shuck-split, when fruit tissue is first exposed, until 60 days after full bloom, which coincided with initiation of pit hardening. In terms of fruit growth, this early-season epidemic coincided with the first stage of stone fruit development, physiologically characterized as the period of cell division. No change in disease levels occurred during midseason, which coincided with the second stage of fruit development, a period of slow growth. However, disease levels declined significantly during the 20 to 30 days prior to harvest, which coincided with the third stage of fruit growth, the period of cell enlargement and epidermal coloration. These disease reductions may be related to physical changes in fruit size and pigmentation causing less established lesions to become undetectable. Based on results of the temporal analysis, we conclude that control measures, consisting of a total of two to four fungicide applications, need only be applied during the early-season phase of the epidemic.

Induction of Growth Promotion and Resistance Against Downy Mildew on Pearl Millet (Pennisetum glaucum) by Rhizobacteria. S. Niranjan Raj, G. Chaluvaraju, K. N. Amruthesh, and H. S. Shetty, De­partment of Studies in Applied Botany and Biotechnology, University of Mysore, Manasagan­gotri, Mysore, India 570006; and M. S. Reddy, and Joseph W. Kloepper, Department of Entomology and Plant Pathology, Alabama Agricultural Experiment Station, Auburn University, Auburn, AL 36849. Plant Dis. D-2003-0203-03R, 2003 (online). Accepted for publication 7 November 2002.

Pearl millet (Pennisetum glaucum) is one of the world’s main coarse grain crops and is grown in India as a rainfed or irrigated crop on 11.8 million ha. Downy mildew caused by the biotrophic oomycetous fungus Sclerospora graminicola (Sacc.) J. Schröt. causes an annual loss of up to 40%, amounting to $270 million annually. The available management strategies have their own limitations; therefore, induction of resistance using plant growth-promoting rhizobacteria (PGPR) was examined in the present study. Seven PGPR strains (Bacillus pumilus T4, B. pumilus INR7, B. amyloliquefaciens IN937a, B. subtilis IN937b, B. pumilus SE34, Brevibacillus brevis IPC11, and B. subtilis GB03) obtained from the culture collection of the Department of Entomology and Plant Pathology, Auburn University, were tested as suspen­sions of fresh suspensions or powdered formulations for growth promotion and management of downy mildew. All treatments with fresh suspensions and powdered formulations showed enhancement in germina­tion and vigor index over the respective controls. With fresh suspensions, maximum vigor index resulted from treatments by B. pumilus strain INR7 followed by B. subtilis strain IN937b (64 and 38% higher than the control, respectively). With powdered formulation, treatment with strain INR7 also resulted in the highest germination and vigor index (10 and 63%, respectively, over the control). Under experimental plot conditions, prominent enhancement in growth also was observed in the disease tests. The powdered formulations showed a striking growth pro­motion when compared with the control. A significant yield enhancement of 40 and 37% over the control was shown by seed treatment with strains INR7 and SE34, respectively, as powdered formulations. The same strains also increased yield by 36 and 33%, re­spectively. Studies on downy mildew management resulted in varied degrees of protection by the PGPR under both greenhouse and field conditions. With fresh suspensions, treatment with INR7 resulted in the high­est protection of 57% followed by B. pumilus strain SE34 and B. subtilis strain GBO3, which resulted in 50 and 43% protection, respectively, compared with the control. PGPR strain INR7 suppressed downy mildew effectively, resulting in 67% protection, while SE34 resulted in 58% protection, followed by GBO3 with 56% protection. Treatment with Apron (Metalaxyl) resulted in the highest protection against downy mildew under both greenhouse and field conditions. Thus, the present study suggests that the tested PGPR, both as powdered formulations and fresh suspensions, can be used within pearl millet downy mildew management strategies and for plant growth promotion.

The Detection and Variation of Strawberry mottle virus. J. R. Thompson and W. Jelkmann, BBA, Institut für Pflanzenschutz im Obstbau, Schwabenheimer Straße 101, D-69221, Dossenheim, Germany. Plant Dis. D-2003-0128-01R, 2003 (online). Accepted for publication 4 November 2002.

Strawberry mottle virus (SMoV) is probably the most important virus to infect strawberry (Fragaria spp.), having been found wherever strawberry is cultivated. All species of strawberry are susceptible to SMoV. Reductions in fruit yield and runner production in single infections can be as high as 30%. In mixed infections with other aphid-transmitted viruses, losses can be up to 80%. However, due to the absence of definitive symptoms in commercial varieties, the only effective means of detecting and identifying SMoV, until now, was by transmission by grafting or aphid transmission to susceptible indicator plants. Such methods are labor intensive, time-consuming, and not 100% reliable. In this work, we describe a method for the rapid and reliable detection of SMoV in Fragaria spp. using a reverse transcription-polymerase chain reaction (RT-PCR) method. This method detected the virus in 16 virus isolates from various geographical origins, including Chile, the Czech Republic, Germany, Holland, Poland, and the United States. Sequence data were also obtained for other genomic regions of the virus isolates, including the putative large coat protein and polymerase genes, thereby providing valuable information on the genomic variability of this virus species and enabling further refinements in the detection method.

Distribution of Pathotypes of Rhynchosporium secalis and Cultivar Reaction on Barley in Alberta. K. Xi, Alberta Agriculture, Food and Rural Development, 6000 C & E Trail, Lacombe, AB T4L 1W1; T. K. Turkington, Agriculture and Agri-Food Canada, Lacombe Research Centre, 6000 C & E Trail, Lacombe, AB T4L 1W1; J. H. Helm, Alberta Agriculture, Food and Rural Development, 5030 50 Street, Lacombe, AB T4L 1W8; K. G. Briggs and J. P. Tewari, Dept. of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, AB T6G 2P5; T. Ferguson, Agricore, 11111 Barlow Trail S.E., Calgary, AB T2C 4M5; and P. D. Kharbanda, Alberta Research Council, Vegreville, AB T9C 1T4. Plant Dis. 2003-0217-01R, 2003 (online). Accepted for publication 5 November 2002.

Barley (Hordeum vulgare) production in Alberta averaged around 6 million metric tons from 2 million hectares annually from 1995 to 1999, and accounted for nearly 50% of the total barley production in western Canada. Scald, caused by Rhynchosporium secalis, is one of the major foliar diseases in Alberta because of intensive barley production, and cool and wet environmental conditions that favor disease development. Yield losses due to scald are estimated to be 1 to 19% in field trials, and losses as high as 20 to 36% have been reported. Average reductions in barley yield can result in annual monetary losses of approximately $7 million in Alberta. In the current study, 44 barley accessions and commercial cultivars with different levels of resistance were evaluated for scald reaction caused by R. secalis from 1997 to 1999 at various sites in Alberta. The accessions Hudson, Atlas, Atlas 46, Atlas 68, Abyssinian, and Kitchin with major resistance genes were resistant to pathotypes of R. secalis at all sites. Although scald levels were low for these accessions, they were significantly different among years. Pathotypes of R. secalis and environmental conditions affected disease levels on the 32 commercial cultivars, resulting in significantly different scald reactions among sites and seasons. The differences in scald reactions that occurred among locations for the commercial cultivars and localization of pathotypes demonstrated in a previous greenhouse study indicated an uneven distribution of pathotypes of R. secalis in Alberta. The resistance in commercial cultivars AC Stacy, Kasota, and Seebe held up at most sites. Most of the cultivars were intermediate to moderately susceptible. Cultivars that were previously considered resistant were intermediate in reaction and became increasingly susceptible at some sites, but not at others, from 1997 to 1999. The sites where cultivars became increasingly susceptible were more diverse in pathogen virulence compared with those sites where the same cultivars were resistant. The considerable variability of R. secalis in response to cultivar resistance in Alberta requires ongoing monitoring of the development and shifts of pathotypes in commercial fields. Furthermore, as new cultivars are registered each year, the commercial cultivars evaluated in the present study may not represent the newer genotypes grown in Alberta. As a consequence, the scald reaction of newly registered cultivars should be evaluated over a range of sites and years. Further studies using more accessions with additional genes may also be needed to facilitate ongoing assessment of pathotype variation and identification of novel sources of resistance.

Molecular Analyses of Citrus tristeza virus Subisolates Separated by Aphid Transmission. R. H. Brlansky, Professor, University of Florida, Citrus Research and Education Center, Lake Alfred; V. D. Damsteegt, Research Plant Pathologist, USDA, ARS, FDWSRU, Fort Detrick, Frederick, MD; and D. S. Howd, Senior Biological Scientist, and A. Roy, Postdoctoral Plant Pathologist, University of Florida, Citrus Research and Education Center, Lake Alfred. Plant Dis. D-2003-0214-03R, 2003 (online). Accepted for publication 25 November 2002.

Citrus tristeza virus (CTV) is the causal agent of one of the most destructive diseases of citrus. Numerous biological strains of CTV exist and cause different symptoms such as stem pitting, decline and death of sweet orange and grapefruit on sour orange rootstock, seedling yellows, vein clearing, and vein corking. These various symptoms also can occur on different citrus hosts. CTV exists in field trees as a mixture of different biological virus types. This mixture or complex may therefore contain both mild and severe CTV. Using the brown citrus aphid as a tool, we have found that different parts (subisolates) of this mixture can be separated. We studied what is transmitted from three different CTV isolates using aphid transmissions and tested these components with serological and molecular analysis. We showed the transmission of severe CTV from mild virus sources. This has implications for regulatory agencies. Understanding what constitutes various CTV field isolates is important in devising methods to quickly and specifically identify CTV strains.

Sensitivity of Watermelon Cultigens to Ambient Ozone in North Carolina. Gerald J. Holmes, Department of Plant Pathology, North Carolina State University, Raleigh 27695, and Jonathan R. Schultheis, Department of Horticultural Science, North Carolina State University, Raleigh 27695. Plant Dis. D-2003-0218-02R, 2003 (online). Accepted for publication 11 November 2002.

Ozone (O(3)) is an important component of the earth’s atmosphere. In the upper atmosphere (15 to 50 km above the earth’s surface), a natural layer of O(3) absorbs harmful ultraviolet radiation. However, at the earth’s surface, O(3) is the most prevalent and damaging air pollutant to which plants are exposed. Plant sensitivity to O(3) is dependent on a number of factors, including species and cultivar. However, little information is available on the relative sensitivity of cultivars within most plant species. Watermelon foliage is sensitive to O(3), causing a distinctive chlorosis and necrosis. During the 2000 and 2001 growing seasons, 93 cultivars and breeding lines (i.e., cultigens) were evaluated for foliar injury caused by naturally occurring levels of O(3). Injury developed on all cultigens in both years, but severity of injury varied among cultigens and between years. Visible foliar injury (percent surface area necrotic or chlorotic) was rated 1 week after the first harvest in both years. Mean injury for all cultigens was 39% in 2000 (range = 16 to 66%) and 20% in 2001 (range = 2.5 to 60%). This corresponded to greater mean seasonal O(3) levels in 2000 (58 ppb) than in 2001 (52 ppb). Seedless cultigens were less sensitive to injury than seeded cultigens. Mean injury for all seedless cultigens was 31% in 2000 and 16% in 2001, whereas injury for seeded cultigens was 47% in 2000 and 25% in 2001. Injury level was weakly correlated to several cultigen characters. Higher injury levels consistently correlated to fewer days-to-harvest. Correlations between injury level and yield, fruit size, fruit shape, and fruit sweetness were inconsistent between years and/or insignificant. Watermelon growers and plant breeders can use this information in efforts to reduce injury due to O(3) in watermelon production areas of high O(3) pollution.

Effects of Light, Temperature, and Leaf Wetness Duration on Daylily Rust. D. S. Mueller and J. W. Buck. University of Georgia, Department of Plant Pathology, Georgia Station, Griffin 30223. Plant Dis. D-2003-0218-01R, 2003 (online). Accepted for publication 19 November 2002.

Daylily rust, caused by the fungus Puccinia hemerocallidis, has become an increasing problem for daylily growers. Determining how environmental factors such as temperature and moisture affect this disease is a key step to developing control measures for this new disease. In laboratory assays, an increase in light intensity significantly decreased rust urediniospore germination. Optimal germination temperature was between 22 and 24°C at low light levels. Once plants were infected there were significantly fewer lesions at 10°C and no lesions developed at 36°C within 15 days. Five to six h of leaf wetness were required for lesion development. These studies indicate that for disease development of P. hemerocallidis on daylily, temperatures at approximately 22°C and 5 h leaf wetness are required during infection. However, once a daylily plant is infected with rust, disease development is not as sensitive to environmental conditions.