Ornamental Plants

Annual Reports and Research Reviews

Evaluation of Crabapples for Diseases at the Secrest Arboretum in Wooster, Ohio: 1996

Summary

Crabapples in a replicated plot at the Secrest Arboretum of The Ohio State University were evaluated for apple scab three times in 1996. Ten of the crabapple selections exhibited no scab at any of the ratings, while 16 of the selections exhibited scab rated as significantly damaging to plant aesthetics for at least one of the ratings. Apple scab was less severe in the plot in 1996 than in 1995, when 20 selections had ratings of significantly damaging scab. The same crabapple selections were also rated for scab on one date at Holden Arboretum and results were similar but with some differences.

Other diseases noted included bacterial fireblight, frogeye leaf spot, sooty blotch, and flyspeck. Frogeye leaf spot at Secrest Arboretum was considerably more severe than in previous years, resulting in some defoliation and significantly affecting plant aesthetics for two crabapple selections, and at least moderate effects on aesthetics for 11 additional selections.

Introduction

Apple scab (pathogen: Venturia inaequalis) is a major fungal disease problem of many crabapple species (Malus spp). Although it generally is not a major health problem for the tree, it can severely impact ornamental effect and the marketability of highly susceptible crabapples.

Symptoms of apple scab on crabapple include olive to gray to brown to black spots on foliage, yellowing and discoloration of foliage, leaf drop, and scabby lesions on fruits. Apple scab can be effectively controlled with a fungicide spray program, and certain cultural and sanitary practices (thinning to avoid dense canopies, cleanup of leaves at the end of the season) are also beneficial for control.

However, the best method for control of apple scab is through the use of genetically resistant crabapple selections. The evaluations presented here are the latest in a series of apple scab evaluations for Ohio (1,2,3,).

The authors emphasize that apple scab in particular and diseases and pests in general are not the only consideration relative to crabapple effectiveness in the landscape. This is the rationale for the inception of more comprehensive evaluations of a number of different aesthetic criteria. These include fruit, flower and foliage features, plant texture and shape, and disease and pest problems. These are reported in a series of publications from data collected in the Secrest plot (4,5,6,7). The comprehensive crabapple evaluations at Secrest are a continuing project and are being expanded to include a second plot with additional selections.

Materials and Methods

Forty-five crabapples in the replicated crabapple plot at Secrest Arboretum were rated for apple scab disease on June 10, 1996; July 2, 1996; and August 9, 1996. This plot is in a completely randomized design with three replications of each crabapple selection (except for 'Red Jewel' and M. zumi 'Calocarpa,' each with two surviving replicates). The plot was planted in 1984 and is not treated with fungicides or insecticides.

Apple scab evaluations were based on the following rating system:

0 = No scab noted.

1 = Slight scab; less than 5% of leaves affected; no negative effect on aesthetics.

2 = Moderate scab; 5%-20% of leaves affected; some yellowing; little or no defoliation; moderate negative effect on aesthetics.

3 = Extensive scab; 20%-50% of leaves affected; significant defoliation and/or leaf yellowing; significant negative effect on aesthetics.

4 = Heavy scab; 50%-80% of leaves affected; severe defoliation and discoloration of leaves; severe negative effect on aesthetics.

5 = Extreme scab; 80%-100% of foliage is affected, and defoliation is complete or nearly complete.

Scab on crabapple fruits was factored into the overall scab ratings.

The same rating system was also used for frogeye leaf spot evaluations in 1996.

Frogeye leaf spot (pathogen: Botryosphaeria obtusa) is typically of minor concern relative to plant health and aesthetics, but on some selections in 1996, it caused significant leaf spotting, yellowing, and some defoliation. Severity of frogeye was greater than that noted in previous years of evaluations in the plot (2,3).

Fireblight (pathogen: Erwinia amylovora) is a serious bacterial disease causing blossom blight, twig and branch dieback, leaf discoloration, and in extreme cases total plant death. Fireblight was not a problem on most crabapple selections in this plot in 1996.

Sooty blotch (pathogen: Gloeodes pomigena) and fly speck (pathogen: Microthyriella rubi) caused considerable fruit unsightliness on several crabapple selections in the plot in 1996.

Results and Discussion

Spring rains during leaf emergence and development in 1996 provided environmental conditions conducive to scab development, although scab was not as severe as in 1995. In general, scab ratings were similar to four-year averages. Early wet conditions and possible overwintering of the frogeye pathogen on infested fruit mummies may have contributed to increased frogeye leaf spot in 1996.

Of the 45 selections in this evaluation plot, 10 exhibited no scab in 1995 (Table 1). Of these 10, M. baccata 'Jackii,' 'Bob White,' 'Prairifire,' 'Red Jewel,' M. sargentii , and 'Strawberry Parfait' also had excellent overall aesthetic qualities and good resistance to other diseases. 'Dolgo,' 'Silver Moon,' and 'White Angel' were scab-free but had mediocre to poor overall aesthetic ratings. 'Silver Moon' also had some fireblight incidence, and 'Beverly' had significant frogeye leaf spot disease.

Scab ratings in this study were only for the 45 selections in the plot at Secrest Arboretum and at Holden Arboretum. This was done because of the replications in this plot as well as to avoid reporting data for just one plant and to facilitate monthly ratings of the Secrest Arboretum plot (4,5). However, there is additional data available for other Ohio locations and for other crabapple selections and locations for previous years in other publications (1,8,9).

In 1995-1996, ratings of crabapples at both Secrest Arboretum and Holden Arboretum near Mentor, Ohio, in northern Ohio near Lake Erie, were made in order to compare scab incidence for these two different environmental areas.

Results for Holden Arboretum in 1996 are included in Table 1 and indicate that scab incidence is generally similar for the two sites.

Frogeye leafspot was present on all crabapples in 1996, except for 'Sentinel,' and had significant negative effects on aesthetics on two crabapples ('Beverly,' 'Royalty') and moderate effects on aesthetics on 11 other crabapples in 1996.

Bacterial fireblight was present on five of the 45 selections in 1996 - 'Harvest Gold,' 'Mary Potter,' 'Silver Moon,' 'Red Jade,' and 'Winter Gold.' Fireblight was not severe in 1996 on any of the crabapple selections, even 'Harvest Gold' and 'Winter Gold,' which had hundreds of blossom blight infections leading to short shoot infections in 1995 (3).

Two additional disease problems noted the past two years are sooty blotch and fly speck, which resulted in overall smudging and ineffectiveness of ornamental fruit display. Significant aesthetic problems with these diseases were noted in 1996 on the following crabapple selections: 'Harvest Gold,''Professor Sprenger,' 'White Angel,' and 'Winter Gold.'

Powdery mildew and rust diseases were completely absent in this plot for 1993-1996. Insect problems in the plot included apple-and-thorn skeletonizer, Japanese beetle, spotted tentiform leafminer, and fall webworm. Ratings for apple-and-thorn-skeletonizer and Japanese beetle infestations are included in a companion article in this Circular (10).

Literature Cited

1. Smith, E. M. and Treaster, S.A. 1992. Evaluation of flowering crabapple susceptibility to apple scab in Ohio - 1991. The Ohio State University and Ohio Agricultural Research and Development Center, Special Circular 140. Ornamental Plants - 1992: A Summary of Research.

2. Chatfield, J.A. and Draper, E. A. 1995. Evaluation of crabapples for apple scab at the Secrest Arboretum in Wooster, Ohio: 1993-1994. The Ohio State University and Ohio Agricultural Research and Development Center, Special Circular 142. Ornamental Plants: A Summary of Research 1993-1994. pp. 10-12.

3. Chatfield, J. A. and Draper, E. A. 1996. Evaluation of crabapples for apple scab at the Secrest Arboretum in Wooster, Ohio: 1995. The Ohio State University and Ohio Agricultural Research and Development Center, Special Circular 142. Ornamental Plants: A Summary of Research, 1993-1994. pp. 22-26.

4. Draper, E. A. and Chatfield, J.A. 1995. Aesthetic evaluation of crabapples at Secrest Arboretum in Wooster, Ohio: 1993-1994. The Ohio State University and Ohio Agricultural Research and Development Center, Special Circular 142. Ornamental Plants: A Summary of Research, 1993-1994. pp. 1-9.

5. Chatfield, J. A., Draper, E.A, Cochran, K.D., Bristol, P.W., and Tubesing, C.E. 1996. Aesthetic evaluation of crabapples at Secrest Arboretum in Wooster, Ohio: 1994-1995. The Ohio State University and Ohio Agricultural Research and Development Center, Special Circular 152. Ornamental Plants: A Summary of Research, 1995. pp. 12-21.

6. Chatfield, J.A., Draper, E.A., and Cochran, K.D. 1996. Comprehensive aesthetic evaluations of crabapples in Ohio: 1993-1995. Malus: International Ornamental Crabapple Bulletin 10(1) 5-16.

7. Draper, E.A., and Chatfield, J.A. 1996. Magic of Malus. American Nurseryman. August 15, 1996. 184 (4) pp.

8. den Boer, John H. and Green, T. L. 1995. Crabapple disease and aesthetic ratings. Malus: International Ornamental Crabapple Bulletin 9(2). pp. 8-56.

9. Pecknold, P.C. 1996. Evaluating crabapple resistance to apple scab under epidemic conditions. Malus 10(1). 35-36.

10. Shetlar, D.J., Chatfield, J.A., Draper, E.A., and Cochran, K.D. 1997. Evaluation of crabapples for apple-and-thorn skeletonizer and Japanese beetle feeding damage at Secrest Arboretum in Wooster, Ohio: 1996. The Ohio State University and Ohio Agricultural Research and Developmental Center, Special Circular 154. Ornamental Plants: Annual Reports and Research Reviews, 1996. pp. 49-52.