Interpretive Summaries

Inheritance of Resistance to Verticillium dahliae in Diploid Interspecific Potato Hybrids. Shelley Jansky, Professor, Department of Biology, University of Wisconsin-Stevens Point, 54481; D. I. Rouse, Professor, Department of Plant Pathology, University of Wisconsin-Madison 53706; and P. J. Kauth, Department of Biology, University of Wisconsin-Stevens Point. Plant Dis. D-2004-0727-01R, 2004 (online). Accepted for publication 6 May 2004.

Verticillium wilt is a serious disease in potato and is caused primarily by the soilborne fungi Verticillium dahliae and V. albo-atrum. Currently, the most common control practice is soil fumigation, which is costly and environmentally harmful. Host plant resistance offers an effective, long-term control strategy for the management of this disease. Although commercially significant cultivars are not resistant, high levels of resistance have been identified in hybrids between cultivated and wild potato lines. In this study, we determined that resistance in these clones is simply inherited. Two genes that interact with each other appear to be responsible for resistance. Therefore, high levels of resistance should be relatively easy to transfer to the cultivated potato.

Viability of Sirococcus clavigignenti-juglandacearum Conidia on Exoskeletons of Three Coleopteran Species. J. E. Stewart, S. Halik, and D. R. Bergdahl, The Rubenstein School of Environment and Natural Resources, Aiken Center, University of Vermont, Burlington 05405. Plant Dis. D-2004-0727-04R, 2004 (on-line). Accepted for publication 21 May 2004.

Butternut canker, caused by the fungus Sirococcus clavigignenti-juglandacearu, is threatening the survival of butternut (Juglans cinerea L.), an eastern hardwood tree. Butternut is valuable economically for its beautiful light-colored wood and ecologically, since the nuts are a major source of food for wildlife. Butternut canker was first reported in 1971, and in 30 years, caused as much as 90% mortality of butternut throughout its range from southeastern Canada to South Carolina and from Minnesota to the eastern seaboard. The fungus produces spores in a sticky matrix, especially during wet conditions. Spores are released by rain splash and may be carried in mist droplets by wind. These wind-disseminated spores are known to travel as much as 40 m from infected trees. Spore dissemination via rain splash and wind is a random event, thus, other means of dissemination are thought to be involved in the rapid spread of S. clavigignenti-juglandacearum throughout the range of butternut. Therefore, it is thought that animals, such as birds, squirrels, or insects that specifically target butternut, may be involved in long distance dispersal of the fungus. In the summers of 2001 and 2002, three species of beetles (Astylopsis macula, Eubulus parochus, and Glischrochilus sanguinolentus) were collected, rinsed, and then artificially infested with spores of S. clavigignenti-juglandacearum. The length of time that beetles carried viable spores and the total numbers of spores carried, up to 16 days, were determined. Field-collected beetles were also examined with scanning electron microscopy. Spores have been observed on the thorax, abdomen, and legs of A. macula and E. parochus. All three species carried viable spores for up to 16 days. Both A. macula and E. parochus carried over one million spores of S. clavigignenti-juglandacearum during various time intervals. A. macula carried the greatest number of conidia (62,000) after 16 days. G. sanguinolentus carried the least number of spores, ranging from 94,000 spores at time 0 to 340 conidia at day 16. These data suggest that all three species have potential to vector spores of S. clavigignenti-juglandacearum; however, A. macula and E. macula may act as more effective vectors. It is still unclear what role these insect species play in the disease cycle of butternut canker. However, the rapid rate of spread of S. clavigignenti-juglandacearum throughout the range of butternut suggests that some means of long-distance dissemination must exist. If beetles associated with butternut can carry viable conidia of S. clavigignenti-juglandacearum for long periods of time, this increases the potential for new infections to occur since these beetles preferentially seek butternut for feeding and oviposition. A more complete understanding of the lifecycles of these species will allow us to better determine the vector role each species may play in the spread of S. clavigignenti-juglandacearum. This knowledge will allow forest managers and forest health specialists to reduce spread of the butternut canker fungus by controlling populations of insect vectors.

Effect of Prohexadione-Calcium Dose Level on Shoot Growth and Fire Blight in Young Apple Trees. J. L. Norelli and S. S. Miller, USDA-ARS, Appalachian Fruit Research Station, 45 Wiltshire Rd., Kearneysville, WV 25430. Plant Dis. D-2004-0802-01R, 2004 (online). Accepted for publication 24 May 2004.

The purpose of this research was to develop a management strategy to control fire blight in young apple orchards using prohexadione-calcium (Phd-Ca). Phd-Ca is a plant growth regulator that suppresses shoot growth and fire blight in apple. In mature orchards, Phd-Ca is effective in managing fire blight. However, in young apple orchards, there are conflicting requirements to control fire blight and allow sufficient tree growth for tree establishment, and the utility of Phd-Ca in young orchards was unclear. When Phd-Ca was applied to orchard-grown apple trees ranging in age from newly planted to fifth season of growth (4-year-old orchards), it was found that two applications of Phd-Ca at 125 mg·liter(^-1) provided a better balance between fire blight control and growth in young orchards than did three or more applications of 63 or 30 mg·liter(^-1). Although the high rate of Phd-Ca suppressed early-season shoot growth more than the lower rates, trees that received the high rate of Phd-Ca tended to grow more in the latter part of the season, resulting in little or no difference in total seasonal growth between trees that received two high- or three low-rate applications of Phd-Ca. Fire blight control with Phd-Ca required shoot growth suppression early in the growing season, and high-dose treatment with Phd-Ca often provided significantly better fire blight control than treatment at lower rates. Poor fire blight control occurred when the rate of Phd-Ca was lowered sufficiently to allow greater early-season growth. The results indicate that one to two Phd-Ca applications of 125 mg·liter(^-1) can be used to manage fire blight in the fourth to sixth season of growth when there is a high risk of shoot blight.

Genes for Adult-Plant Resistance to Leaf Rust in Soft Red Winter Wheat. Yeshi A. Wamishe and Eugene A. Milus, Department of Plant Pathology, University of Arkansas, Fayetteville 72701. Plant Dis. D-2004-0809-01R, 2004 (online). Accepted for publication 26 May 2004.

Leaf rust caused by Puccinia triticina is an important and widespread disease of wheat that can cause significant yield loss. Resistance to leaf rust has been the most cost effective and environmentally safe means of managing the disease. Adult-plant resistance is a type of resistance whereby seedlings are susceptible but plants become more resistant as they mature. The objective of this study was to determine the genetic basis for adult-plant resistance in contemporary soft red winter wheat lines and to determine the best methods for identifying genes Lr12, 13, and 34, which confer adult-plant resistance. Of the 116 lines evaluated, more than 90% expressed adult-plant resistance when inoculated with a race of the pathogen that attacked seedlings. Genes Lr12, 13, and 34 were identified in 17, 23, and 27 of the lines, respectively. Lr12 was best identified by a distinctive reaction on flag leaves when inoculated plants were incubated under controlled conditions. Although Lr13 is considered a gene for adult-plant resistance, it was best identified by inoculating seedlings with particular isolates of the pathogen that can overcome this gene at 18.1°C but not at 25.5°C. Lr34 was best identified by evaluating lines in irrigated field plots for leaf tip necrosis, a trait known to be tightly linked to Lr34. More than 40% of the lines had adult-plant resistance that could not be attributed to genes Lr12, 13, or 34. The results of this study indicated that adult-plant resistance to leaf rust is common among contemporary soft red winter wheat lines and that genes in addition to Lr12, 13, and 34 contribute to the resistance. Furthermore, certain methods for identifying particular genes were found to be more effective than other published methods, but none of the methods were definitive.