The responses of 12 soybean cultivars to Sclerotinia sclerotiorum were evaluated under field and controlled environment conditions. The 12 cultivars were planted in fields naturally or artificially infested with S. sclerotiorum and evaluated for disease incidence. In controlled environment studies, the cultivars were compared with respect to lesion size on detached leaves and stems inoculated with mycelial disks, incidence of stem rot following mycelial inoculation of foliage, lesion lengths on stems discolored by oxalic acid, and levels of soluble pigment(s) in stems. Significant differences (P ≤ 0.05) in disease incidence, lesion sizes, and levels of soluble pigment(s) were detected among the 12 cultivars in all evaluations, but ranking of cultivars varied among methods and experiments within methods. Corsoy 79 and S19-90 were consistently most resistant in all methods; whereas Kenwood, A2242, Bell, and Williams 82 were least resistant. Pearson correlation coefficients (R) for disease incidence between location years in field experiments (FE) ranged from 0.86 to 0.95. R values between FE and controlled environment experiments (CEE) ranged from 0.01 to 0.62 for detached leaf assays (DLA), -0.20 to 0.47 for lesion lengths on stems inoculated with mycelial disks (LLM), 0.38 to 0.45 for incidence of stem rot from mycelial inoculation of foliage (MIF), 0.08 to 0.66 for lesion lengths on stems discolored by oxalic acid (LLO), and -0.55 to -0.37 for levels of soluble stem pigment(s) (SSP). Absolute values of R between FE and CEE were 0.40 or greater in 40, 17, 33, 29, and 83% of all correlation analyses for DLA, LLM, MIF, LLO, and SSP, respectively. Spearman's coefficients of rank correlation (r s) between FE and CEE based on average performance were 0.55, -0.20, 0.40, 0.42, and -0.44 for DLA, LLM, MIF, LLO, and SSP, respectively. Disease development was slow when foliage was inoculated with a mycelial suspension. Based on experiment (E) by cultivar (C) interaction and rs values between replicate experiments within each method, determination of soluble stem pigment levels (0.86 ≤ rs ≤ 0.97; P < 0.001 for 100% of six analyses; P = 0.98 for E × C interaction) and measurement of lesion lengths on stems discolored by oxalic acid (0.27 ≤ rs ≤ 0.85; rs ≥ 0.58 and P ≤ 0.05 for 68% of 28 analyses; P = 0.07 for E × C interaction) were the most repeatable methods. The results from this study suggest that determination of levels of soluble pigments in stems, measurement of lesion lengths on stems discolored by oxalic acid, and the detached leaf assay may be better than mycelial inoculation of stems or foliage in evaluating soybean cultivars for field resistance to S. sclerotiorum.

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

Molecular markers at 103 loci were used to identify the location of quantitative sources of resistance to Exserohilum turcicum in 150 F2∶3 lines of a B52/Mo17 maize population. Host-plant response was measured in terms of the average number of lesions per leaf, the average percent leaf tissue diseased (severity), and the average size of lesions. The location of quantitative trait loci were compared with three loci having known qualitative effects, namely Ht1, Ht2 and bx1. Chromosomal regions containing the Ht1 and Ht2 loci showed a small contribution in determining lesion size, even though alleles with dominant, qualitative effects at these loci have never been reported in either inbred parent. Similar effects were not observed for the number of lesions or for disease severity. Likewise, some contribution was observed for chromosomal regions encompassing the bx1 locus in determining lesion size but not the number of lesions or disease severity. Overall the contribution of loci in the vicinity of Ht1, Ht2 and bx1 was small relative to variation attributable to loci with quantitative effects identified in this study. Molecular-marker-facilitated mapping concurred with previous reciprocal translocation mapping studies on the importance of chromosomes 3, 5 and 7, despite the fact that these studies utilized diverse sources of resistant germplasm.

. 2005. Effects of wounding and inoculation with Sclerotinia sclerotiorum on isoflavone concentrations in soybean. Can. J. Plant Sci. 85: 749-760. Isoflavones play an important role in the defense response of soybean to pathogen attack. They are involved in nodulation of legumes and are associated with human health benefits including the prevention of heart disease and cancers. Concentrations of the isoflavones daidzein, genistein, and glycitein, the glucoside conjugates daidzin, genistin and glycitin, the acetyl glucoside conjugates acetyldaidzin, acetylgenistin, and acetylglycitin, and the malonyl glucoside conjugates malonyldaidzin, malonylgenistin, and malonylglycitin were determined in ovendried leaves of 12 soybean cultivars whose stems were (i) non-wounded, non-inoculated (NWNI), (ii) wounded, non-inoculated (WNI), and (iii) wounded and inoculated (WI) with mycelia of Sclerotinia sclerotiorum, the causal fungus of Sclerotinia stem rot of soybean. There were significant differences (P ≤ 0.05) among cultivars in concentrations of isoflavones and their conjugates in all wounding treatments. Concentrations of the aglycones daidzein and genistein were higher (P ≤ 0.05) in WI than in WNI and NWNI plants in all cultivars. Glycitein and its conjugates were detected only in some cultivars in much lower concentrations than daidzein and genistein and their conjugates. Concentrations of total daidzein in the 12 cultivars ranged from 68 to 491, 174 to 781, and 282 to 553 µg g -1 dry weight in NWNI, WNI, and WI plants, respectively. Concentrations of total genistein ranged from 128 to 427, 290 to 840, and 296 to 759 µg g -1 dry weight in NWNI, WNI, and WI plants, respectively. Concentrations of total glycitein ranged from 0 to 44, 0 to 13, and 0 to 24 µg g -1 dry weight in NWNI, WNI, and WI plants, respectively. In NWNI plants, the cultivar Corsoy 79 ranked in the top two (rank 1 = highest concentration, rank 12 = lowest concentration) in concentrations of daidzein and genistein and their conjugates except genistin (ninth rank). In WNI plants, Parker ranked first in concentrations of all conjugates of daidzein and genistein whereas Corsoy 79 consistently ranked in the top four in concentrations of the two isoflavones and their conjugates. In WI plants, Parker ranked first in concentrations of all conjugates of daidzein and genistein except acetyldaidzin (second rank), S19-90 ranked in the top five in concentrations of all conjugates of daidzein and genistein, whereas Corsoy 79 ranked in the top five in concentrations of daidzein and genistein and all of their conjugates. The results from this study suggest that soybean cultivars differ in concentrations of constitutive or induced isoflavones and in their ability to accumulate isoflavones following wounding and/or infection by S. sclerotiorum. For personal use only.

RFLPs were used to investigate components of host-plant response to Exserohilum turcicum in 150 unselected F2∶3 lines of a B52/Mo17 maize population. Following inoculation with spore suspensions of the pathogen (race 0), components of disease development were measured and then quantitative trait mapping was performed to identify the location and effects of quantitative trait loci (QTLs) determining host-plant response. Components of interest were the average number of lesions per leaf, the average percent leaf tissue diseased (severity) and the average size of lesions (cm(2)). Based on a LOD threshold of 2.31 (P<0.05), the number of lesions appears to be associated with QTLs on chromosomes 1S, 3L, 5S. Severity was associated with analogous regions and, in addition, QTLs on chromosomes 7L and 8L. Most QTLs, for either of these two components, involve additive gene action and partial dominance or overdominance. In contrast, lesion size was associated with QTLs on chromosomes 7L and 5L; recessive gene action may be involved at 7L.

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Wheat streak mosaic and maize dwarf mosaic viruses were isolated from a number of maize (Zea mays L.) plants in seed production fields in Iowa. A low percentage of seed transmission (0.1% for wheat streak mosaic virus and 0.2% for maize dwarf mosaic virus in one line) of the two viruses was found. Results from mechanical inoculation of several maize inbred lines showed that both viruses could be recovered from some symptomless plants. The Ht gene does not impart susceptibility to wheat streak mosaic virus.

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