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The Magic of Malus

James A. Chatfield and Erik.A. Draper

Associate Professor, Ohio State University Extension and
Assistant Professor, Ohio State University Extension

A paper from the Proceedings of the 12^th Metropolitan Tree Improvement Alliance Conference (METRIA 12), Landscape Plant Symposium: Plant Development And Utilization, held in Asheville, NC, May 23-25, 2002, co-sponsored by the North Carolina State University, North Carolina Division of Forest Resources, USDA Forest Service Southern Region, North Carolina Landscape Association, North Carolina Association of Nurserymen, The Landscape Plant Development Center, The North American Branch of The Maple Society, and The International Ornamental Crabapple Society.

Introduction

The International Ornamental Crabapple Society (IOCS) was established in 1986 to evaluate ornamental crabapples (Malus taxa) for apple scab and other diseases and ornamental aesthetics as well as to promote crabapples for diverse environmental and landscape design situations. As part of the IOCS program a National Crabapple Evaluation Program was set up throughout the United States, with 17 replicated trial plots currently part of the program. This article details some of the key findings in the past decade at one of those plots, namely the Crablandia plots at the Secrest Arboretum at the Ohio Agricultural Research and Development Center of the Ohio State University in Wooster, Ohio

At Secrest Arboretum the Crablandia I plot consisted of 46 crabapple taxa with three single plant replicates planted in a completely randomized design. Crablandi I was established in 1983. The Crablandia II plot currently consists of 63 crabapple taxa with five single plant relpicates planted in a completely randomized design. Crablandia II was established in 1998. Three examples of data collected from these plots are presented and discussed below.

Aesthetic Evaluations

In both Crablandia I and Crablandia II, crabapples were evaluated monthly for aesthetic characteristics, from September 1992 through 2000 in Crablandia I and from 2000-2002 in Crablandia II. Detailed results of these aesthetic ratings are presented elsewhere (1-2), but from these monthly ratings full seasonal profiles have been developed for crabapple taxa in the plots, providing information on the full range of ornamental characteristics of crabapples, from flower and fruit to foliage and form. Following are a short list of fifteen diverse crabapple taxa with examples of short profiles developed from these aesthetic ratings. This is not a Top 15 list as such lists are too limiting, but rather these 15 examples reflect the diversity of crabapple taxa and the range of characteristics considered in the profiles.

¹Time of Effective Fruit Display derived from observations conductly monthly throughout the year. Effective fruit impact is defined as the period from the tree's fruit first contributes to tree aesthetics until the fruit is no longer ornamental.

Apple Scab At Secrest Arboretum in 2001

Apple scab (pathogen: Venturia inaequalis) is recognized by everyone as a key disease problem of ornamental crabapples, on some taxa resulting in significant foliage and fruit symptoms. Following is an example of one year of apple scab evaluations at Secrest Arboretum. Apple scab pressure was high at the Secrest Arboretum in 2001. Yet, even under this considerable disease pressure, 20 of the 63 taxa showed no evidence of apple scab in 2001 and a total of 30 never received a rating that exceeded 1 (no aesthetic impact) on any evaluation date. Twenty-one taxa received a rating of 3 or higher on at least one date in 2001, indicating substantial defoliation and aesthetic impact (Table 2).

Sixty three crabapple taxa were planted in 1997-1998 in Crablandia II at the Secrest Arboretum in a completely randomized design. There were five replicate plants for each taxa with the exception of ‘Brandywine', ‘Canary', ‘Dolgo', ‘Indian Magic', ‘King Arthur' and ‘Royal Scepter', for which there were four replicates, and ‘Hamlet', for which there were three. Plants were mulched with composted yard waste and irrigated as needed during the year of transplanting. Weeds were controlled with spot applications of glyphosate. On 13 June, 9 July, 2 August, and 19 September 2001, all trees were rated on a scale of 0-5, with 0 = no scab observed; 1 = less than 5% of leaves affected and no aesthetic impact; 2 = 5-20% of leaves affected, with some yellowing but little or no defoliation, moderate aesthetic impact; 3 = 20-50% of leaves affected, significant defoliation and/or leaf yellowing, substantial aesthetic impact; 4 = 50-80% of leaves affected, severe foliar discoloration and defoliation, severe aesthetic impact; and 5 = 80-100% of foliage affected, with 90-100% defoliation.

Apple scab ratings of crabapples at Secrest Arboretum for the 2001 season are presented in Table 2. Some key findings in 2001 included:

• A third of the crabapple taxa in the plot exhibited significant defoliation and substantial negative aesthetic effect in 2001 (Table 2).
• Despite this level of scab pressure, nearly a third of the crabapple taxa in the plot exhibited no apple scab incidence in 2001 (Table 2). Furthermore, nearly half of the crabapples in the plot did not exhibit aesthetic impact from apple scab in 2001. This data, coupled with similar data from previous years. suggests that landscape managers have the option of selecting a wide palette of crabapples with little or no scab problems.
• For the second year in row, ‘Prairifire' crabapple did exhibit some scab in the plot, though it was minor. This is of some concern because scab was not seen on ‘Prairifire' for several decades in the current and a former plots at Secrest Arboretum. Though the level of scab was minor on ‘Prairifire', its presence in 2000 and 2001 raises the question of whether there is a new race of the apple scab fungus (Venturia inaequalis) in the arboretum.

*0 = no scab observed.
1 = less than 5% of leaves affected and no aesthetic impact.
2 = 5-20% of leaves affected, with some yellowing but little or no defoliation, moderate aesthetic impact.
3 = 20-50% of leaves affected, significant defoliation and/or leaf yellowing, substantial aesthetic impact.
4 = 50-80% of leaves affected, severe foliar discoloration and defoliation, severe aesthetic impact.
5 = 80-100% of foliage affected, with 90-100% defoliation.

**Means with the same letter in a column are not significantly different (LSD test, p<0.05).

Apple Scab Evaluations From 1993-2000

One of the questions that is often asked of plant pathologists is to predict how bad a disease will be in an upcoming season. Plant pathologists usually hedge a great deal, though sometimes predictions are made based on how bad the disease was in a previous season, with inferences made relative to the future based, for example, on the probable amount of overwintering inoculum available for infection the next year. Relative to this and relative to apple scab in particular, the following graph illustrates the dangers of making such predictions. Table 3 indicates the amount of apple scab disease (based on the 0-5 scale described above) on the 46 crabapple taxa in Crablandia I from 1993-2000. The numbers are based on the August scab ratings in the plots, averaged over all of the taxa. Of particular note, see the years 1999 and 2000. In 1999 the relatively dry spring and summer weather in Wooster, Ohio resulted in the second lowest average scab in the plot over the 8 year period of the study. In 2000, the unusually wet spring and early summer weather in Wooster, Ohio resulted in the highest average scab in the plot in the 8 years of the study. To try to predict in the winter of 2000 how much scab would occur in the upcoming growing season, based on the low level of scab in 1999 would have been futile, since the then unknown upcoming environmental conditions in 2000 were the trump card.

Table 3. Apple Scab on Crabapples at Secrest Arboretum: 1993-2000

References

1. Draper, E.A., Chatfield, J.A., Cochran, K.D. 2000. Crabapples at Secrest Arboretum-Aesthetics and More: A 2000 Update. Ornamental Plants Annual Reports and Research Reviews, 2000, pp.73-86.
2. Chatfield, J.A., Draper, E.A., and K.D. Cochran. 1996. Comprehensive aestehtic evaluations of crabapples in Ohio: 1993-1995. Malus 10(1) 5-16.
3. den Boer, John H. 1995. Blossom Times. Malus- The International Ornamental Crabapple Society Bulletin. 9(1). pp. 10-16.
4. Fiala, Father John L. 1994. Flowering Crabapples - The Genus Malus. Timber Press, Portland Oregon.

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