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Dep. Plant and Soil Sciences, Kentucky Agricultural Experiment Station, Univ. of Kentucky, Lexington, KY 40546-0091. Journal article no. 07-06-087 of the Kentucky Agric. Exp. Stn., Lexington, KY. Published with approval of the director
* Corresponding author (nltaylor{at}uky.edu).
ABSTRACT
‘FreedomMR’ red clover (Trifolium pratense L. 69-L-38-1921, Reg. No. CV-29, PI 651017) was developed by the Kentucky Agricultural Experiment Station and released in 2006. It was selected for mildew resistance (caused by Erysiphe polygoni DC em Salm.) from ‘Freedom!’. FreedomMR, like Freedom!, has reduced pubescence (nonglandular hairs). FreedomMR is a medium red clover cultivar adapted to Kentucky and the central clover region of the United States. It is especially recommended for hay because it less dusty than ‘Kenland’ and more mildew resistant than Freedom! or Kenland.
‘FreedomMR’ red clover (Trifolium pratense L. 69-L38-1921; Reg. No. CV-29, PI 651017) was derived from the cultivar Freedom! (PI 629112; Taylor and Collins, 2003) by five cycles of phenotypic recurrent selection for resistance to powdery mildew caused by Erysiphe polygoni DC em Salm. FreedomMR possesses the same general characteristics and is adapted to the same general area of as Freedom! and ‘Kenland’ (PI 300150; Hollowell, 1951), the central clover region of the United States. It differs from Freedom! in possessing a higher level of mildew resistance and slightly more pubescence. Forage yields as tested for several years in Kentucky were similar to Freedom! and Kenland. However, quality of FreedomMR may be somewhat higher because of its mildew resistance.
Methods
Recurrent Selection Process
Plants of Freedom! (Taylor and Collins, 2003) were subjected to five generations of selection for mildew resistance starting in 2000. Approximately 10,000 plants of each generation were germinated in a greenhouse at Lexington, KY, in January and exposed to natural infections of powdery mildew. Susceptible plants were eliminated, and survivors in each year were transplanted to fields near Lexington. The percentage of plants retained as parents for each subsequent generation ranged from 60.0% in year 2000 to 2.3% in year 2005. Additional susceptible plants eliminated from field-grown populations ranged from 8.7% in year 2001 to none in years 2002 to 2005. Resistant plants were allowed to intercross in isolation from other red clover plants. Seeds were threshed with a combine and cleaned with standard seed-cleaning equipment as needed. This seed was germinated for the next cycle of selection, which was completed in 2005. Plants of year 2005 were maintained in a field for an additional year to allow selection for absence of pubescence.
Evaluation of Selected Generations
An evaluation study to determine effectiveness of selection for mildew resistance was sown in a greenhouse on 8 Feb. 2006. The five generations of selection plus the check cultivars Kenland, Kenstar, Freedom!, Kenway, Kenton, and Marathon and two experimentals, KY Tetraploid, and Wisconsin W1-2 were sown at a rate of 200 seeds per row in trays containing a mixture of equal parts of Pro mix (Premier Horticultural, Inc., Quakertown, PA), sand and Maury silt loam soil. Seeds were treated with Arasan fungicide (E. I Du Pont de Nemours, and Co., Wilmington, DE) to prevent damping off due to unknown causes. The statistical design was a randomized complete block with four replications. When plants reached a height of about 20 cm (15 May), they were visually rated for degree of natural infection with powdery mildew, based on a 1 to 10 score (1 = least infection, 10 = most infection). Data were analyzed by analysis of variance and linear regression. Ten plants of each row from the greenhouse experiment were transplanted to a field (0.92 m squares) in four replicate randomized block design near Lexington on 20 July 2006 to evaluate changes in pubescence occurring during selection for mildew resistance. On 5 Sept. 2006 and 4 June 2007, pubescence was evaluated visually using a scale of 1 to 9 (1 = only a few hairs on the first internode below the flowering head, 9 = the presence of very dense pubescence throughout the length of the stem) (Taylor and Collins, 2003). Pubescence data were analyzed by analysis of variance. Duncan's multiple range test (p = 0.05) was used to differentiate means of mildew infection and pubescence.
Forage Yield
FreedomMR was evaluated for forage yield in four trials at Lexington, Princeton, Quicksand, and Eden Shale experiment station farms in Kentucky in 2006. These stations sample a wide range of environmental conditions in Kentucky. The soils at Princeton (Crider [fine-silty, mixed, active, mesic Typic Paleudalfs]), Lexington (Maury [fine, mixed, semiactive, mesic Typic Paleudalfs]), Quicksand (Pope [coarse-loamy, mixed, active, mesic Fluventic Dystrudepts]), and Eden Shale (Nicholson [fine-silty, mixed, active, mesic Oxyaquic Fragiudalfs]) were silt loams well suited to clover production. Plots 1.5 by 4.6 m were arranged in a randomized complete block design with four replications and sown at 13.4 kg ha–1 into a prepared seedbed using a disk drill. The first cutting of the seeding year was delayed until full bloom, but other harvests were taken when the clover was in bud to early-bloom stage using a sickle-type forage harvester. Fresh weight samples were taken at each harvest to calculate dry matter production. All tests for establishment, fertility, and harvest management were managed according to University of Kentucky Cooperative Extension Service recommendations. Weeds were controlled to avoid limiting production and persistence (Olson et al., 2006). Data were analyzed by analysis of variance and statistical differences among means were differentiated by LSD tests at 0.05% level.
Results
Mildew Resistance
Greenhouse selection was effective reaching a peak in mildew resistance by four cycles with a rating of 1.3, compared with Freedom! at 5.8 and Kenland at 9.0 (Table 1
). The gain in resistance per cycle was 0.94 according to linear regression analysis. The regression of scores over cycles (R2) was only 0.31 because cycle 2 was scored as more susceptible than cycle 1 or the base population, Freedom! The Wisconsin cultivar Marathon and the experimental W1-2 scored 1.3 and 1.0, respectively, and were approximately equal to cycle 4 of the Freedom! selection. These data suggest that the mildew races in Wisconsin and Kentucky are similar if not identical. The apparent increase in mildew susceptibility of Kentucky cultivars, therefore, was likely a result of increased aggressiveness of the present races. The reason for this is unknown.
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Forage Yield and Stand
Forage yield trials at four locations in Kentucky over 2 or 3 yr (Olson et al., 2006) comparing FreedomMR with Freedom! and Kenland showed no differences among cultivars at Lexington, Quicksand and Owenton (Table 2
). At Princeton FreedomMR yielded significantly less than Freedom! but was not different from Kenland. In agreement with the yield data, FreedomMR maintained significantly less stand than Freedom! only at Princeton. These data indicate that selection for mildew resistance had little or no deleterious impact on forage yield and stand. Mildew infection was not recorded in these trials, but the resistance of FreedomMR should result in higher-quality forage.
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This cultivar has been released on an exclusive basis to Barenbrug USA, and it is expected that FreedomMR will replace Freedom! when seed supplies are exhausted. Breeder seed will be maintained by the Foundation Seed Project, Agriculture Experiment Station, University of Kentucky. Limited quantities of seed are available for testing purposes from the author for a period of 5 yr. Recipients of seed are asked to acknowledge the source if FreedomMR is used in the development of a new cultivar, germplasm, parental line, or genetic stock. Foundation and certified seed classes will be permitted beyond Breeder seed. U.S. Plant Variety Protection will not be sought.
Footnotes
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.
Received for publication January 11, 2008.
References
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