Interpretive Summaries

Identification of Magnaporthe oryzae Avirulence Genes to Three Rice Blast Resistance Genes. C. X. Luo, Faculty of Agriculture, Saga University, Saga, 840-8502, Japan; Y. Fujita, National Agricultural Research Center for Hokkaido Region, Sapporo, 062-8555, Japan; N. Yasuda, National Agricultural Research Center, Tsukuba, 305-8666, Japan; K. Hirayae, National Agricultural Research Center, Joetsu, 943-0193, Japan; T. Nakajima, National Agricultural Research Center for Kyushu Okinawa Region, Kumamoto, 861-1192, Japan; N. Hayashi, National Institute of Agrobiological Sciences, Tsukuba, 305-8602, Japan; and M. Kusaba and H. Yaegashi, Faculty of Agriculture, Saga University, Saga, 840-8502, Japan. Plant Dis. D-2003-1216-01R, 2004. Accepted for publication 9 October 2003.

Rice blast, caused by Magnaporthe oryzae, is the most disastrous and widespread disease of rice. One of the most effective means of controlling rice blast is through the use of resistant rice cultivars, but newly developed resistant cultivars often lose their resistance after a few years of commercial production. The breakdown of resistance in rice cultivars to blast is probably due to genetic instability of avirulence (Avr) genes. To identify a genetic mechanism that governs the instability of Avr genes corresponding to the resistance (R) genes would be indispensable for developing rice cultivars with lasting resistance. To identify the Avr genes that correspond to the specific R genes Pik, Piz, and Piz-t, we determined the mode of inheritance of avirulence on three race-differential rice cultivars through a genetic cross of rice blast isolates and then identified the Avr genes based on the reaction of F3 families of rice to both the parental isolates and standard isolates harboring known Avr genes. We also constructed a partial linkage map containing the Avr genes by using random amplified polymorphic DNA (RAPD) analysis.

Patterns of Virulence Diversity in Puccinia triticina on Wheat in Egypt and the United States in 1998-2000. D. V. McVey, United States Department of Agriculture-Agricultural Research Service (USDA-ARS), Cereal Disease Laboratory, St. Paul, MN 55108; M. Nazim, Faculty of Agriculture, Minufiya University, Shibin el-Kom, Egypt; K. J. Leonard and D. L. Long, USDA-ARS, Cereal Disease Laboratory, St. Paul, MN. Plant Dis. D-2003-1222-01R, 2004 (online). Accepted for publication 7 October 2003.

Wheat, an important food crop in Egypt, is damaged by frequent leaf rust epidemics. Lack of knowledge of where rust spores come from each year to infect wheat in Egypt and limited information on races of leaf rust in Egypt made breeding resistant wheat cultivars difficult. We collected samples of leaf rust in 1998 to 2000 from Egypt and from suspected source countries for leaf rust epidemics: Israel, Sudan, Turkey, and Romania. We tested them for virulence on wheat lines with 20 different genes for resistance to identify leaf rust races. We characterized patterns of diversity of leaf rust races in Egypt and other countries and compared them with patterns found in seven regions of the United States. Similarity of virulence patterns showed that leaf rust spores commonly move between Egypt and Israel but rarely spread into Egypt from Sudan, Turkey, or Eastern Europe. Common races recurred in Egypt in successive years, which means that the rust fungus must survive through the summer in Egypt to re-infect newly planted wheat each winter. This year-to-year survival accounts for natural selection of more virulent races in Egypt to overcome race-specific resistance used in Egyptian wheat cultivars. If the oversummering sites for wheat leaf rust in Egypt can be identified, the cycle of selection of more virulent races can be broken and yield losses to leaf rust can be controlled.

Effect of Sclerotinia Stem Rot on Yield of Soybean Inoculated at Different Growth Stages. G. A. Danielson and B. D. Nelson, Department of Plant Pathology, and T. C. Helms, Department of Plant Sciences, North Dakota State University, Fargo 58105. Plant Dis. D-2004-0112-02R, 2004. Accepted for publication 24 October 2003.

Sclerotinia stem rot is an important disease of soybean in the north-central region of the United States. To understand the impact of this disease on production, it is necessary to know the yield losses the disease can cause. This research measured the effect of Sclerotinia stem rot on seed weight, seed numbers, pod numbers, and oil and protein content of plants inoculated at two growth stages. The effects of disease on yield were variable. Disease resulted in significant seed weight loss, with reductions per diseased plant ranging from 19 to 39%. The estimated yield loss per 10% disease incidence ranged from 83 to 229 kg/ha, with an average loss of 137 kg/ha for four field experiments. A reduction in the number of seeds and pods per plant and seed oil content occurred in some, but not all, experiments. Seed protein was not affected. Inoculation at the beginning of flowering compared with 2 weeks later had a similar effect on yield.

Differences in Etiology Affect Mefenoxam Efficacy and the Control of Pink Rot and Leak Tuber Diseases of Potato. Raymond J. Taylor, Bacilio Salas, and Neil C. Gudmestad, Department of Plant Pathology, North Dakota State University, Fargo 58105. Plant Dis. D-2004-0114-01R, 2004 (online). Accepted for publication 18 November 2003.

Pink rot and leak are important soil-borne diseases of potato caused by Phytophthora erythroseptica and Pythium ultimum, respectively. These pathogens differ in the way they infect potato tubers. Pythium ultimum requires a wound for infection to occur, while P. erythroseptica is able to directly penetrate undamaged tuber tissue and cause disease. Although certain cultural practices may be employed to control these “water rot” diseases, mefenoxam is the only effective fungicide currently available. Mefenoxam concentrates in the periderm and outer tissue layers of the potato tuber. Any damage to the tuber could disrupt this chemical barrier, allowing a wound pathogen such as P. ultimum an unprotected infection site. This study was undertaken to examine the factors affecting the development of pink rot and leak in potato tubers and the efficacy of mefenoxam in controlling these diseases. Mefenoxam was applied to replicated field plots in-furrow at planting, in-furrow at planting with a follow-up sidedress, as a single foliar application when tubers were 7 to 8 mm in diameter, and as two foliar applications when the tubers were 7 to 8 mm in diameter and 14 days later. Several different application rates were used with each method. The effectiveness of the fungicide was determined after harvest by challenge-inoculating healthy tubers with the pathogens. Mefenoxam was more effective in controlling pink rot relative to leak over all application methods. The greatest level of pink rot control (89%) was attained with the in-furrow at planting and sidedress application. This method provided only a modest level of leak control (35%). Foliar applications of mefenoxam resulted in 10 to 50% control of pink rot, but leak was not controlled by either foliar application method. The use of mefenoxam to control pink rot appears to be well founded. Since the combination of mefenoxam applied in-furrow and as a banded sidedressing after emergence was the only method that reliably controlled leak, serious consideration should be given to this method when planning control strategies for this disease.