Ornamental Plants

Annual Reports and Research Summaries

Special Circular 152-96

Evaluation of Crabapples for Apple Scab at Secrest Arboretum in Wooster, Ohio: 1995

Erik A. Draper
James A. Chatfield
Kenneth C. Cochran
Peter W. Bristol
Charles E. Tubesing

Erik A. Draper, Ohio State University Extension-Mahoning County; James A. Chatfield, Ohio State University Extension-Northeast District/Horticulture and Crop Science; Kenneth C. Cochran, Ohio Agricultural Research and Development Center/Secrest Arboretum/ Ohio State University Agricultural Technical Institute; Peter W. Bristol, Holden Arboretum; and Charles E. Tubesing, Holden Arboretum

Abstract

Crabapples in a replicated plot at the Secrest Arboretum of The Ohio State University's Ohio Agricultural Research and Development Center were evaluated for apple scab four times in 1995. Ten of the crabapple selections exhibited no scab at any of the ratings; 20 of the selections exhibited scab rated as extensive and significantly damaging to plant aesthetics for at least one of the ratings. Apple scab was more severe in the plot in 1995 than in 1994, when 11 selections had ratings of significantly damaging scab. The same crabapple selections also were rated on one of the dates at Holden Arboretum and results were similar but with some major differences. Other diseases noted included bacterial fireblight, frogeye leaf spot, sooty blotch and flyspeck.

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 affect the ornamental value and 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 fungicidal spray program. Certain cultural and sanitary practices, such as thinning to avoid dense canopies and 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 (1,2,3,4,).

The authors emphasize that apple scab in particular and diseases and pests in general are not the only considerations 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 companion paper to this apple scab evaluation report (5,6).

Materials and Methods

Forty-five crabapple selections in the Secrest Arboretum were rated for apple scab disease on June 8, 1995; July 7, 1995; August 7,1995; and September 7, 1995. Crabapples in the Secrest trial are arranged in a completely randomized design with three replications. Trees were planted in 1984 and are not treated with pesticides.

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% to 20% of leaves affected; some yellowing; little or no defoliation; moderate negative effect on aesthetics.

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

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

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

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

Other diseases noted in the plot included frogeye leafspot, fireblight, and sooty blotch and fly speck of fruits.

Frogeye leaf spot (pathogen: Botryosphaeria obtusa) is typically of minor concern relative to plant health and aesthetics, but on some selections causes significant leaf spotting, yellowing, and occasionally defoliation.

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

Sooty blotch (pathogen: Gloeodes pomigena) and fly speck (pathogen: Microthyriella rubi) cause considerable smudginess of apple fruits and in 1995 caused considerable fruit unsightliness on several crabapple selections in the plot.

Further comments on these diseases and on aesthetics of all 45 crabapples selections in the plot at Secrest Arboretum are presented in detail in the companion article in this Circular, "Aesthetic Evaluations of Crabapples at Secrest Arboretum in Wooster, Ohio: 1995."

Results and Discussion

Spring and early summer weather of sustained rainy periods with moderate temperatures resulted in heavy scab pressure in 1995. In virtually all cases the scab ratings on the crabapple selections in the Secrest plot were greater in 1995 than for the previous two years and greater than the three-year (1993-1995) average scab ratings (see Table 1). A scab rating of 3 (extensive scab; 20% to 50% of leaves affected; significant defoliation and/or leaf yellowing; significant negative effect on aesthetics) occurred for at least one rating period on 20 selections in 1995, compared to 11 in 1994.

Of the 45 selections in this evaluation plot, there were 10 that exhibited no scab in 1995. Of these 10, M. baccata 'Jackii,' 'Bob White,' 'Prairifire,' 'Red Jewel,' Malus sargentii, and 'Strawberry Parfait' also had excellent aesthetic qualities and good resistance to other diseases. 'Beverly,' 'Dolgo,' 'Silver Moon,' and 'White Angel' were scab-free but had mediocre to poor overall aesthetic ratings. 'Silver Moon' also had significant fireblight problems.

Another 12 selections in the plot exhibited apple scab ratings under 2 for 1995, and 20 selections had average scab ratings under 2 for the 1993-1995 period. A rating of 2 in the author's rating scheme is the point at which moderate negative effects on overall plant aesthetics begin to play a role in ornamental effectiveness. Some of these selections with relatively low apple scab have other problems as effective landscape ornamentals. In addition, some of the selections with scab ratings of 2 or greater have redeeming ornamental features for some situations, at least for a portion of the year (5,6).

Scab ratings in this study were only for the 45 selections in the plot at Secrest Arboretum. This was done because the plot is replicated (6,7). However, there is additional data available for other locations and additional crabapple selections (1,2,3,4,7).

Of particular interest is the fall 1995 edition of Malus (7), which compiled data for the past 30 years from crabapple collections throughout the United States. In many cases, data in 1995 at Secrest Arboretum was similar to the combined results reported in the Malus article, but there were a number of major differences.

These include:

• On the basis of Secrest data from 1993-1995, 'Adams.' 'Red Splendor,' Selkirk,' 'White Cascade,' Centurion,' 'Donald Wyman, 'Harvest Gold,' 'Henningii,' 'Jewelberry,' 'Profusion,' 'Molten Lava,' 'Ormiston Roy,' 'Professor Sprenger,' 'Red Jade,' 'Snowdrift,' 'Sugar Tyme,' and '(Weeping) Candied Apple' would not be identified as "highly resistant," as they are in the Malus article.

• On the basis of Secrest data, 'Royalty,' 'Indian Magic,' 'Indian Summer,' 'Ralph Shay,' 'Red Barron,' 'Ruby Luster,' 'Winter Gold,' and 'Velvet Pillar' would not be identified as "resistant," as they are in the Malus article.

There are numerous possible explanations for the differences in these reports. Following are two possibilities. First, scab may differ at other locations due to either less or more favorable environmental conditions for scab development. Second, there may be local races of Venturia inaequalis in certain locations.

In 1995, companion ratings of crabapples at both Secrest Arboretum and Holden Arboretum in Kirtland, Ohio, in northern Ohio near Lake Erie, were made in order to compare scab incidence for these two locations. Comparative data for these locations is presented in Table 2. Ratings for the two arboreta were generally similar, though scab was less severe at Holden Arboretum.

Crabapple selections with considerably lower scab at Holden included 'Red Splendor,' 'White Cascade,' 'Selkirk,' M. zumi 'Calocarpa,' 'Ormiston Roy,' and M. floribunda. Of greatest interest was the total absence of scab on 'Red Splendor' on 7-6-95 at Holden, while it exhibited moderate scab incidence at Secrest on 7-7-95.

Frogeye leafspot was present on most crab-apples, but was aesthetically significant (leaf spotting, yellowing, and moderate defoliation) on only one selection in 1995, 'Professor Sprenger.'

Bacterial fireblight was present on six of the 45 selections -- 'Harvest Gold,' 'Mary Potter,' 'Ormiston Roy,' 'Silver Moon,' 'Red Jewel,' and 'Winter Gold.' Fireblight was severe in 1995 only on 'Harvest Gold' and 'Winter Gold,' on which hundreds of blossom blight infections spread into relatively short (4" to 6" long) blighting of spur shoots, resulting in browning and blight of over 25 percent of the overall foliage of affected plants. 'Harvest Gold' and 'Winter Gold' in the plot that did not bloom this year (in the alternate year of their bloom cycle) did not exhibit fireblight, highlighting the importance of blossom infections in the fireblight disease epidemiology.

Two crabapple selections were removed from the plot this year due to complete death or near death from disease. These were Malus tschonoskii due to fireblight and 'Madonna' due to fireblight and frogeye leaf spot.

Two additional disease problems noted this year included sooty blotch and fly speck, which resulted in overall smudging and ineffectiveness of ornamental fruit display. Significant aesthetic problems with these diseases was noted in 1995 on the following crabapple selections: 'Professor Sprenger,' 'Red Jade,' and 'White Angel.'

Powdery mildew and rust diseases were completely absent in this plot for 1993-1995. Insect problems in the plot included apple thorn skeletonizer, Japanese beetle, spotted tentiform leafminer, and fall webworm, but ratings for individual crabapple selections were not made for 1993-1995, although this may be included in future trials.

Literature Cited

1. Brewer, J. E., L. P. Nichols, C. C. Powell, and E. M. Smith. 1979. The Flowering Crabapple -- A Tree for All Seasons. Cooperative Extension Service of Northeast States. NE 223, NCR 78.

2. Smith, Elton M. 1979. A 10-Year Evaluation of Flowering Crabapple Susceptibility to Apple Scab in Ohio. Ohio Agricultural Research and Development Center. Research Circular 246. Ornamental Plants -- 1979: A Summary of Research. pp.36-39.

3. Smith, Elton M. and Sharon A. Treaster. 1992. Evaluation of Flowering Crabapple Susceptibility to Apple Scab in Ohio -- 1991. Ohio State University and Ohio Agricultural Research and Development Center Special Circular 140. Ornamental Plants -- 1992: A Summary of Research.

4. Chatfield, James A. and Erik A. Draper. 1995. Evaluation of Crabapples for Apple Scab at the Secrest Arboretum in Wooster, Ohio: 1993-1994. Ohio State University and Ohio Agricultural Research and Development Center Special Circular 142. Ornamental Plants: A Summary of Research 1993-1994. pp. 10-12.

5. Draper, Erik A., and James A. Chatfield. 1995. Aesthetic Evaluation of Crabapples at Secrest Arboretum in Wooster, Ohio: 1993-1994. Ohio State University and Ohio Agricultural Research and Development Center Special Circular 142. Ornamental Plants: A Summary of Research, 1993-1994. pp. 1-9.

6. Chatfield, James A., Erik A. Draper, Kenneth C. Cochran, Peter W. Bristol, and Charles E. Tubesing. 1996. Aesthetic Evaluation of Crabapples at Secrest Arboretum in Wooster, Ohio: 1994-1995. Ohio State University and Ohio Agricultural Research and Development Center Special Circular 152. Ornamental Plants: A Summary of Research, 1995. pp. 12-21.

7. den Boer, John H. and Thomas L. Green. 1995. Crabapple Disease and Aesthetic Ratings. Malus: International Ornamental Crabapple Bulletin 9(2). pp. 8-56.