The apple industry throughout the world has been shifting from the large, “standard” tree, with its massive, non-economic cambial surface, and its attendant massive machinery, to much more compact units. Orchard densities have changed from 65 trees/ha to densities of 350 to 2000 trees/ha.
After a severe fire blight epiphytotic, fruiting trees of rootstock cultivars were scored for kill of fruit spurs, for lesion extension, and total tree damage. Spur infections were heaviest on ‘M 9’, ‘M 12’, ‘M 16’, ‘M 25’, ‘M 26’ and ‘Rotyp’. Damage to ‘M 9’ and ‘M 26’ trees was especially severe. Least affected were ‘M 2’, ‘M 7’, ‘M 15’ and a Malus prunifolia selection, PI 286613. Intermediate susceptibility was indicated for ‘M 27’ and a red-leaved introduction from the USSR, PI 274842.
Progenies from 48 small-fruited Malus species and cultivars were tested for heritable, high-level resistance to powdery mildew [Podosphaera leucotricha (Ell. & Ev.) Salm.]. Six-week-old seedlings from controlled crosses and open pollination were inoculated by dusting with conidia in the greenhouse. ‘David’, ‘White Angel’, M. × robusta (Robusta 5), M. × robusta ‘Korea’, and an unnamed selection M. × robusta (24-7-7,8) all produced some seedlings in their progenies that remained free of infection after inoculation. The mode of inheritance of resistance was not clear.
Resistance to apple scab (Venturia inaequalis) in apple cultivar breeding has been derived mainly from the Vf gene from Malus floribunda 821, which introgresses horticulturally unfavorable characters. M. sieversii, now thought to be the primary progenitor of M. × domestica, grows wild in many diverse habitats in Central Asia and can have fruit quality comparable to commercial cultivars. Since 1989, four major collections of M. sieversii have been made in Central Asia, where scab is endemic. Some seed collections have been made from trees with superior fruit, that were not infected with scab. Over a 6-year period, 3000 seedlings from 220 wild M. sieversii trees representing 10 diverse ecosystems in Kazakstan, Uzbekistan, Kyrgyzstan, and Tajikistan have been inoculated with conidia of five races and two wild types of V. inaequalis. Suspensions (270,000 conidia/ml) were applied to 4- to 8-leaved seedlings, which were incubated for 48 h at 19°C with constant leaf wetness. Symptoms for three resistant reactions were assessed 2 to 4 weeks after inoculation: A = chlorosis with crinkling (Vf type reaction); B = stellate necrotic lesions (Vr type reaction), and N = large necrotic areas (uncharacterized resistant reaction). Results indicated that nearly 20% of the seedlings showed one or more of the resistant reactions. The range of resistance within seedling populations from each of the 220 single-tree sources ranged from 0% to 75%. Significant differences existed among seedlings from each of the ecosystems. Most resistance reactions appeared to be similar to those observed for Vr from “Russian seedling.” Resistant selections with superior horticultural traits may constitute a genepool for increased efficiency of breeding scab-resistant cvs. This genepool may also be useful to address the breakdown of resistance to V. inaequalis race 6.
A primary focus of the apple rootstock breeding and evaluation program at USDA-ARS/Cornell Univ. has been to develop screening protocols to identify genotypes resistant to the fire blight bacterium (Erwinia amylovora). Direct inoculation is a simple technique, but does not represent the only mode by which rootstocks become infected in the orchard. Selection based on direct inoculation screens may, however, enrich the population for resistant genotypes. Large breeding populations from controlled crosses are shoot-tip inoculated with E. amylovora, and the fraction showing the highest levels of resistance are retained for further evaluations. These survivors are again screened through direct inoculation in the field, and the less-resistant genotypes are discarded. Following selection for other pathogen tolerance and horticultural characters, elite genotypes are multiplied through asexual propagation. Replicated tests using direct inoculation with multiple strains of E. amylovora are then used to estimate the level of fire blight resistance of elite genotypes. A final screen utilizes mature, grafted orchard trees to verify that the resistance of rootstock genotypes to fire blight is maintained under conditions simulating natural infection. Direct inoculation screening and selection have resulted in a high frequency of strong resistance to severe fire blight epidemics in recent orchard inoculation trials.
‘Liberty,’ which was tested under the number NY55140-19 is a good quality red apple that can be grown without fungicidal sprays in New York State (3). It is highly resistant to Venturia inaequalis (Cke.) Wint., apple scab, and to Gymnosporangium juniperivirginianae (Schw.), cedar apple rust and resistant to Podosphaera leucotricha (E. & E) Salm, apple powdery mildew, and Erwinia amylovora (Burrill) Bergey et al, fire blight.
Malus sieversii is one of the primary progenitors of the cultivated apple. Since 1989, several collecting trips have been made to central Asia by personnel of the USDA and Cornell Univ. to collect seeds of wild Malus sieversii from many diverse ecosystems. In 1992, an ex situ plot in Geneva, N.Y., was established with trees grown from seed that was collected in three different habitats in Kazakstan, Tajikistan, and Uzbekistan in 1989. In 1995, trees grown from seed that was collected in five additional habitats in Kazakstan and Kyrgyzstan in 1993 were added to the ex situ plot. In the summers of 1995 and 1996, tips of vigorously growing shoots of 1135 seedlings from 79 different populations were inoculated by hypodermic syringe with 5 × 108 cfu/ml of Erwinia amylovora strain Ea273. Seedlings from the 1989 collection were in the fourth and fifth field-growing seasons, with some beginning to bear fruit. Seedlings from the 1993 collection were in first and second field-growing seasons. Results from both seasons indicated that individuals within each of the 79 populations of M. sieversii are resistant to fire blight (defined as ≤20% shoot length infected). Resistance differed among populations, with some populations having no resistant individuals and others having >80% of the seedlings resistant. The range of resistance is quite similar to that seen among apple cultivars from North America and Europe. In another test, some accessions from 1989 collection had sufficient bloom for inoculation in 1995 and 1996. At full bloom, blossoms on these trees were inoculated with the E.amylovora suspensions (5 × 107 cfu/ml) using a backpack sprayer. These also gave diverse resistant reactions.
Pgm-1, the gene responsible for variation in the most anodal isozyme of phosphoglucomutase in apple (Malus spp.), is shown to lie ≈8 centiMorgans from the gene Vf, which confers apple-scab resistance. The proximity of the marker and the ease by which allozymic forms can be resolved suggest that Pgm-1 will be useful for following the inheritance of scab resistance conferred by Vf.