P. M. Dixon

Using plant EST collections, we obtained 1392 potential gene duplicates across 8 plant species: Zea mays, Oryza sativa, Sorghum bicolor, Hordeum vulgare, Solanum tuberosum, Lycopersicon esculentum, Medicago truncatula, and Glycine max. We estimated the synonymous and nonsynonymous distances between each gene pair and identified two to three mixtures of normal distributions corresponding to one to three rounds of genome duplication in each species. Within the Poaceae, we found a conserved duplication event among all four species that occurred approximately 50-60 million years ago (Mya); an event that probably occurred before the major radiation of the grasses. In the Solanaceae, we found evidence for a conserved duplication event approximately 50-52 Mya. A duplication in soybean occurred approximately 44 Mya and a duplication in Medicago about 58 Mya. Comparing synonymous and nonsynonymous distances allowed us to determine that most duplicate gene pairs are under purifying, negative selection. We calculated Pearson's correlation coefficients to provide us with a measure of how gene expression patterns have changed between duplicate pairs, and compared this across evolutionary distances. This analysis showed that some duplicates seemed to retain expression patterns between pairs, whereas others showed uncorrelated expression.

ABSTRACT Gray leaf spot, caused by the fungus Cercospora zeae-maydis, causes considerable yield losses in hybrid maize grown in the north-central United States and elsewhere. Nonchemical management tactics have not adequately prevented these losses. The probability of profitably using fungicide application as a management tool for gray leaf spot was evaluated in 10 field experiments under conditions of natural inoculum in Iowa. Gray leaf spot severity in untreated control plots ranged from 2.6 to 72.8% for the ear leaf and from 3.0 to 7.7 (1 to 9 scale) for whole-plot ratings. In each experiment, fungicide applications with propiconazole or mancozeb significantly reduced gray leaf spot severity. Fungicide treatment significantly (P </= 0.05) increased yield by as much as 1.65 t/ha with a single propiconazole application. There were significant (P < 0.05) correlations between gray leaf spot severity and yield. We used a Bayesian inference method to calculate for each experiment the probability of achieving a positive net return with one or two propiconazole applications, based on the mean yields and standard deviations for treated and untreated plots, the price of grain, and the costs of the fungicide applications. For one application, the probability ranged from approximately 0.06 to more than 0.99, and exceeded 0.50 in six of nine scenarios (specific experiment/hybrid). The highest probabilities occurred in the 1995 experiments with the most susceptible hybrid. Probabilities were almost always higher for a single application of propiconazole than for two applications. These results indicate that a single application of propiconazole frequently can be profitable for gray leaf spot management in Iowa, but the probability of a profitable application is strongly influenced by hybrid susceptibility. The calculation of probabilities for positive net returns was more informative than mean separation in terms of assessing the economic success of the fungicide applications.

ABSTRACT Strawberry leaves (cv. Tristar) inoculated with Colletotrichum acuta-tum conidia were incubated at 10, 15, 20, 25, 30, and 35 degrees C under continuous wetness, and at 25 degrees C under six intermittent wetness regimes. The number of conidia and appressoria was quantified on excised leaf disks. In order to assess pathogen survival, inoculated leaves were frozen and incubated to induce acervular development. Germination, secondary3 conidiation, and appressorial development were significantly (P </= 0.05) affected by temperature and wetness treatments. Under continuous wetness, the optimum temperature range for conidial germination was 23.0 to 27.7 degrees C, whereas the optimum temperature for appressorial development ranged from 17.6 to 26.5 degrees C. Secondary conidiation showed an optimum temperature range of 21.3 to 32.7 degrees C and was most abundant between 12 and 36 h after inoculation. Conidial germination, appressorial production, and secondary conidiation were favored by increasing wetness duration and more than 4 h of wetness were required for secondary conidiation. In a greenhouse, C. acutatum survived up to 8 weeks on leaves. The number of acervuli formed on leaves after freezing and incubation was closely (r(2) >/= 0.95) related to appressorial populations prior to this treatment and was greatest following periods of continuous wetness. Production of secondary conidia and appressoria of C. acutatum on symptomless strawberry leaves under a range of environmental conditions suggests that these processes also occur under field conditions and contribute to inoculum availability during the growing season.

Previously known only from the southern United States, hosta petiole rot recently appeared in the northern United States. Sclerotium rolfsii var. delphinii is believed to be the predominant petiole rot pathogen in the northern United States, whereas S. rolfsii is most prevalent in the southern United States. In order to test the hypothesis that different tolerance to climate extremes affects the geographic distribution of these fungi, the survival of S. rolfsii and S. rolfsii var. delphinii in the northern and southeastern United States was investigated. At each of four locations, nylon screen bags containing sclerotia were placed on the surface of bare soil and at 20-cm depth. Sclerotia were recovered six times from November 2005 to July 2006 in North Dakota and Iowa, and from December 2005 to August 2006 in North Carolina and Georgia. Survival was estimated by quantifying percentage of sclerotium survival on carrot agar. Sclerotia of S. rolfsii var. delphinii survived until at least late July in all four states. In contrast, no S. rolfsii sclerotia survived until June in North Dakota or Iowa, whereas 18.5% survived until August in North Carolina and 10.3% survived in Georgia. The results suggest that inability to tolerate low temperature extremes limits the northern range of S. rolfsii.

ABSTRACT Conidial suspensions of Colletotrichum acutatum were prepared in 1:27, 1:45, and 1:81 (wt/vol) dilutions of an extract of strawberry (cv. Tristar) flowers or leaves in water. Strawberry leaves and plastic coverslips were sprayed with the conidial suspensions, incubated at 25 degrees C and continuous wetness for 48 h, and the number of conidia and appressoria were counted. In another experiment, leaves and coverslips were sprayed with a conidial suspension in water, incubated for 72 h to establish C. acutatum populations, and placed in a growth chamber under dry conditions for up to 6 weeks. At each sampling time, leaves and coverslips were sprayed with flower extracts, leaf extracts, or water, incubated for 48 h at 25 degrees C and continuous wetness, and the number of conidia and appressoria were counted. Flower extracts significantly (P </= 0.05) increased the number of conidia on leaves and coverslips compared with water, both when conidia were applied with the extracts and when the extracts were applied to C. acutatum populations exposed to dryness for up to 2 weeks. Application of flower extracts resulted in up to 10- and 16-fold increases in conidia on leaves and coverslips, respectively. Number of conidia increased more when exposed to flower extracts than to leaf extracts. Production of appressoria was not significantly affected by flower or leaf extracts. Our results suggest that inoculum levels of C. acutatum on foliage may increase during flowering of strawberry plants.