Interpretive Summaries

Host–Pathogen Interactions Between Phytophthora infestans and the Solanaceous Hosts Calibrachoa × hybridus, Petunia × hybrida, and Nicotiana benthamiana. M. C. Becktell, C. D. Smart, C. H. Haney, and W. E. Fry, Department of Plant Pathology, Cornell University, Ithaca, NY 14853. Plant Dis. DOI: 10.1094/PD-90-0024. Accepted for publication 24 June 2005.

Late blight, caused by the pathogen Phytophthora infestans, is a devastating disease of potato and tomato, but can also damage other plants in the same family (Solanaceae). To understand how P. infestans interacts with hosts other than potato and tomato, we looked at late blight on three different plants (three host–pathogen systems). The first two plants, petunias (Petunia × hybrida) and calibrachoas (Calibrachoa × hybridus), are ornamental plants that are commonly found in greenhouses and gardens. Petunias have been implicated as hosts previously, but not investigated thoroughly; calibrachoas have never been tested as hosts. Both petunias and calibrachoas can be found in greenhouses where tomatoes are also grown; therefore, the susceptibility of these plants is of importance to the greenhouse industry. The third plant, Nicotiana benthamiana, is related to tobacco and is commonly found in plant pathology research labs. In our study, we found that the majority of the petunias tested were susceptible, but that a few cultivars were resistant. The resistant cultivars responded with a type of resistance (differential resistance) that suggests petunias possess specific genes for resistance against some isolates of the pathogen. We considered the age of the petunia plants and whether this might influence resistance as it had in other late blight hosts. We found that as petunias age, they become more resistant to late blight. In the second host–pathogen system involving calibrachoas, we found that six of the 10 cultivars tested were resistant to P. infestans. For the final species, N. benthamiana, we found that this plant was susceptible to all P. infestans isolates tested in our study. Because previous reports had shown that this plant was resistant to P. infestans unless the inf1 gene of the pathogen was mutated, we tested our isolates for the presence of a functional inf1 gene and its associated INF1 protein. We found that all the isolates in our study possessed the inf1 gene and the INF1 protein. The results of our research indicate that the host range of P. infestans is broader than we had originally expected. As hosts to P. infestans, petunias and calibrachoas should be included in disease management considerations when tomatoes are in close proximity. The utility of N. benthamiana as an important model for lab studies is expanded to include studies with P. infestans.

Comparison of Severity Assessment Methods for Predicting Yield Loss to Rhizoctonia Foliar Blight in Soybean. K. C. Stetina, S. R. Stetina, and J. S. Russin, Department of Plant Pathology and Crop Physiology, Louisiana State University Agricultural Center, Baton Rouge 70803. Plant Dis. DOI: 10.1094/PD-90-0039. Accepted for publication 10 July 2005.

Rhizoctonia foliar blight, caused by the fungus Rhizoctonia solani, is a disease that causes significant yield loss in soybean in the Gulf Coast states, especially Louisiana, Arkansas, and Mississippi. Normally, testing soybean lines for resistance is done when disease pressure is very high, but this may not occur in the field every year. A disease-assessment method that would allow reliable evaluation of soybean lines under low- to moderate-disease-pressure conditions could improve the cultivar development process. Yield loss in this study was due primarily to loss of entire pods rather than smaller seed or incomplete pod filling, as occurs with some other diseases and insect pests. Disease ratings based specifically on pod damage successfully identified differences between known resistant and susceptible cultivars and were better than conventional foliar ratings at detecting these differences under low-disease conditions. The pod-based disease rating described in this study will allow soybean breeders to reliably test cultivars for resistance to this disease even in years when disease pressure is low. The resulting timely release of resistant soybean cultivars will benefit soybean growers in this region of the United States.

Effectiveness of an Attenuated Zucchini yellow mosaic virus Isolate for Cross-Protecting Cucumber. Yoshitaka Kosaka, Kyoto Prefectural Institute of Agricultural Biotechnology, Kyoto 619-0244; Bo-Song Ryang, Laboratory of Plant Pathology, Graduate School of Agriculture and Biological Sciences, Osaka Prefecture University, Osaka 599-8531; Takashi Kobori, Kyoto Prefectural Institute of Agricultural Biotechnology, Kyoto 619-0244; Hiroshi Shiomi, Takii Plant Breeding and Experimental Station, Shiga 520-3231; Hisao Yasuhara, Kyoto Biken Laboratories, Inc., Kyoto 611-0041; and Mitsunobu Kataoka, Kyoto Prefectural Institute of Agricultural Biotechnology, Kyoto 619-0244, Japan. Plant Dis. DOI: 10.1094/PD-90-0067. Accepted for publication 14 August 2005.

Zucchini yellow mosaic virus (ZYMV) causes serious damage to cucurbit crops worldwide. In Japan, cucumbers are commonly affected by ZYMV during summer to early autumn. Infected plants exhibit severe mosaic and distortion on leaves and fruits, or wilting in grafted plants. To protect cucumber plants from ZYMV, we tried to develop an attenuated virus as the “vaccine” for preventive inoculations. We obtained a candidate vaccine, designated ZYMV-2002, following cold treatment by maintaining diseased plants at 12.5 to 15°C for about 2 months. Vaccinated cucumber plants had very similar fruit productivity to healthy control plants under field conditions. During field experiments in 2002 and 2003, vaccinated plants significantly suppressed infection with ZYMV, progression of disease severity, and reduction of fruit yield and quality. These results demonstrate that ZYMV-2002 is a potentially useful vaccine for reducing the impact of ZYMV.

Effect of Potato virus Y on Yield of Three Potato Cultivars Grown Under Different Nitrogen Levels. Jonathan L. Whitworth, United States Department of Agriculture–Agricultural Research Service, Aberdeen Research & Extension Center, Aberdeen, ID 83210; Phil Nolte and Chris McIntosh, University of Idaho, Idaho Falls 83402; and Robert Davidson, Colorado State University, Center 81125. Plant Dis. DOI: 10.1094/PD-90-0073. Accepted for publication 23 August 2005.

Potato virus Y (PVY) limits yield in potato even in mild-PVY-symptom cultivars such as Shepody and Russet Norkotah. A common assumption by seed potato growers is that extra nitrogen (N) applied to mild-PVY-symptom cultivars may reduce the PVY yield impact. If extra N fertilizer increases the overall size and appearance of a PVY-infected plant, there may be less yield reduction caused by PVY. Previous work has examined PVY yield reduction, but not the effect of PVY and N level on yield. Knowledge gained from the effect of increased N fertilizer on PVY-infected fields may allow a grower to reduce PVY yield loss. This research examines the yield effect of different nitrogen levels on PVY-positive and PVY-negative potato plants of cvs. Russet Burbank, Russet Norkotah, and CO80011-5 (also known as Crestone Russet). Results show that there is a significant yield reduction between PVY-positive and PVY-negative plants at most nitrogen levels. PVY yield reduction was similar (approximately 38%) in the mild-symptom-expression clones of Russet Norkotah and CO80011-5, whereas the yield reduction in Russet Burbank, which exhibits typical PVY symptom expression, was 63.5%. We conclude that increased nitrogen does not significantly alter yield reduction due to PVY infection.

Lignin Degradation by Fusarium solani f. sp. glycines. V. V. Lozovaya, A. V. Lygin, O. V. Zernova, S. Li, and J. M. Widholm, Department of Crop Sciences, and G. L. Hartman, United States Department of Agriculture–Agricultural Research Service and Department of Crop Sciences, National Soybean Research Center, University of Illinois, Urbana 61801. Plant Dis. DOI: 10.1094/PD-90-0077. Accepted for publication 16 August 2005.

Lignin degradation may play a role in the infection, colonization, and survival of some fungi that attack plants. Sudden death syndrome, caused by the soilborne fungal pathogen Fusarium solani f. sp. glycines, is one of the most important diseases of soybean. Lignin degradation by F. solani f. sp. glycines was shown by several biochemical events. In some cases, the lignin degradation by F. solani f. sp. glycines was intermediate or greater than that found with two known lignin-degrading fungi, Polyporus tulipifera and Schizophyllum commune. These results indicate that F. solani f. sp. glycines was capable of degrading lignin which may be important in the infection process, colonization of root tissue, and in the survival of the fungus.

Pre- and Postharvest Treatments to Control Green Mold of Citrus Fruit During Ethylene Degreening. J. L. Smilanick, United States Department of Agriculture–Agricultural Research Service, San Joaquin Agricultural Sciences Center, Parlier, CA 93648; M. F. Mansour, Department of Horticulture, Menofiya University, Shebin El-Kom, Egypt; and D. Sorenson, Fruit Grower’s Supply Co., Orange Cove, CA 93646. Plant Dis. DOI: 10.1094/PD-90-0089. Accepted for publication 23 August 2005.

Citrus fruit often rot after harvest before they can be sold or used by consumers. Two approaches, fungicide applications to trees before harvest and drenching fruit after harvest, were evaluated to minimize postharvest green mold, caused by the fungus Penicillium digitatum. Green mold is particularly a problem among fruit subjected to ethylene gas after harvest, a practice termed “degreening” that eliminates green rind color. Preharvest applications of the fungicide thiophanate methyl (TM) consistently reduced the number of rotten fruit by about 80%. Postharvest application of the fungicide thiabendazole (TBZ) by drenching harvested fruit in bins before degreening also was very effective. In semicommercial tests with naturally inoculated fruit, TBZ and sodium bicarbonate treatment reduced green mold incidence by about 90%. Neither TM nor TBZ influenced green color removal during degreening of orange fruit, and both are relatively inexpensive.