Disease-resistant apple cultivars

Introduction

There are several insects and diseases that can attack apples grown in Ontario. Apple producers must exercise some control over these problems or fruit losses may be excessive and economic production not possible.

Several cultivars are now available which have high resistance or field immunity to apple scab. Table 1 is a partial list of available scab resistant apple cultivars, and additional selections are presently being developed. Some of these cultivars also have resistance to other diseases (Table 2).

Table 1. Scab Resistant Apple Cultivars.

• PRI - Cooperative introductions from the State Universities in Purdue, Rutgers and Illinois.
• AAFC, St. Jean, PQ - Agriculture and Agri-Food Canada, St. Jean sur Richelieu, PQ
• Agriculture Canada, SEF - Smithfield Experimental Farm, Trenton, ON (closed)
• NYSAES, NY - New York State Agriculture Experimental Station, Geneva, NY
• INRA, France - Institute National de la Recherche Agronomique, Angers, France

Apple scab caused by Venturia inaequalis (Cke.) Wint., is the most serious disease affecting apples. Growing cultivars resistant to apple scab eliminates the need for control of this fungal disease. Apple scab is not the only disease that can hamper apple production in Ontario. Powdery mildew incited by Podosphaera leucotricha (Ell. & Ev.) Salm, and the bacterial disease, fire blight incited by Erwinia amylovora (Burr.) are also of major concern.

Cultivars with resistance to the major early season diseases (apple scab, fire blight, powdery mildew, cedar apple rust) are now under development and a few which are moderately or highly resistant to these diseases have been released. However, combining resistance to several diseases in a new apple cultivar, as well as maintaining desirable fruit characteristics, has been a formidable task. The level of reported disease resistance and the quality of the fruit produced for disease resistant cultivars in outlined in Table 2.

Resistance to other, less prevalent diseases, has not been included in disease resistance breeding programs. Susceptibility of the cultivars listed here to such problems as black rot, sooty blotch, fly speck, quince rust and other diseases remains largely undetermined. Such diseases might limit the adaptablility of otherwise disease resistant cultivars in some locations if no fungicide sprays are applied.

Before a large planting of disease resistant apples is attempted, disease resistance, fruit quality, and most importantly, market acceptability should be evaluated under a grower's own set of circumstances. Because of the wide range of growing conditions in Ontario apple districts, the best way to evaluate these cultivars is to establish your own small planting including those cultivars of interest.

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Figure 1. Liberty apples.

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Figure 2. Freedom apples.

Table 2. Selected cultivars with demonstrated field resistance

to the diseases listed. Not all cultivars listed have been evaluated in Ontario. Where this is the case the information given is taken from the originating source.

Note: Disease Resistance Ratings

R - resistant (no control necessary)
MR - moderately resistant (control may be necessary under severe disease pressure)
S - susceptible (control necessary in growing areas where the disease is prevalent)
U - unknown
Storage - months in refrigerated storage at 1° C.
Best use - processing: juice, sauce or pie fill - dessert: fresh fruit - dual: combined use
Harvest Date - observed or estimated under Ontario conditions