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- Author or Editor: T. van der Zwet x
During 1976-1980, three plant exploration trips were made throughout eastern Europe in search of native Pyrus germplasm. A total of 384 accessions (231 from Yugoslavia, 86 from Romania, 43 from Poland, and 12 each from Hungary and Czechoslovakia) were collected as budwood and propagated at the National Plant Germplasm Quarantine Center in Glenn Dale, Md. Following 8 years of exposure to the fire blight bacterium [Erwinia amylovora (Burr.) Winsl. et al.], 17.49” of the accessions remained uninfected, 11.2% rated resistant, 6.8% moderately resistant, and 64.6% blighted severely (26% to 100% of tree blighted). Some of the superior accessions have been released for use in the pear breeding program.
Abstract
Aerial strands of Erwinia amylovora (Burr.) Winslow et al. the causal organism of fire blight of pear and apple, are a form of ooze produced by infected tissues. Strands contain large num bers of pathogenic bacteria embedded in a matrix. Strands are mainly of two types, smooth or beaded. Smooth strands (Fig. 1A) of about equal diameter throughout their length are formed by a uniform quantity of ooze extruded through natural openings of the epidermis. Upon exposure to air, the ooze solidifies to produce the strand. As more ooze is extruded basipetally, the strand increases in length. Beaded strands (Fig. 1A) areformed by extrusion of the ooze in spurts.
Abstract
Various pear species bloom at different times during the spring. Pyrus calleryana Decaisne is one of the earliest and P. communis L. is one of the latest blooming species. In 1975 there were 34 days between the earliest and latest blooming clones. When species or selections with different bloom times were crossed, two generations were needed to bring the bloom time of the progeny to the bloom time of the later blooming parent. The bloom of any given progeny occurred within only 4 to 5 days. No seedling in a progeny flowered later than the later blooming parent. Discovery of late blooming germplasm is essential for the development of late blooming types of pears.
Abstract
Pear cultivars, previously rated as moderately to highly resistant to fire blight, were subjected to further natural infection, supplemented by artificial inoculation with Erwinia amylovora (Burr.) Winsl. et al. in various tissues. Of 49 cultivars evaluated, 69% remained in the same class or one class below their previous rating. There appeared to be very little correlation between the amount of blossom blight and overall degree of blight resistance in cultivars. Pear cultivars were shown to exhibit varying degrees of blight resistance in succulent shoots and woody trunk tissue.
Abstract
During the early 1900s, several plant exploration trips to the Orient were made to collect Pyrus species from Japan, China, Korea, and Manchuria to search for resistance to fire blight [Erwinia amylovora (Burr.) Winsl. et al.]. Following numerous inoculations in various tree tissues, Reimer (15) reported the four most important species, ranked in descending order of blight resistance, as follows: P. ussuriensis Maxim, P. calleryana Deche, P. betulaefolia Bunge, and P. pyrifolia (Burn.) f. Nak. Pyrus ussuriensis and P. pyrifolia were used in earlier breeding programs and are the parent species of many oriental cultivars. However, their textures and flavors have not been accepted by most Americans. P. calleryana also has been used in the USD A pear breeding program, and selections of the 3rd backcross generation appear useful only for resistance.
Fruit production requires extensive use of pesticides to control pest damage and maintain high product quality. Hydrophobic particles alter the leaf surface due to the hydrophobic and reflective nature of the particles and impart characteristics that make the plant surface incompatible, and/or unrecognizable to the pest. Hydrophobic particles were applied to apple and pear in field and greenhouse studies. Specific diseases, insect pests, plant growth, and yield were monitored and evaluated on treated plants in comparison to untreated and chemically treated controls. Powdery mildew in apple and Fabrea leaf spot in pear were controlled by the hydrophobic particles. Aphids, mites, and psylla were controlled in apple and pear. Hydrophobic clays have the potential of cross-protection for several disease and insect pests while imparting beneficial horticultural effects that would increase long-term productivity and sustainability of fruit production systems.
Abstract
A natural complex hybrid, ‘Pitoma Slanopadja’, between native Pyrus amygdaliformis Vill. and cultivated P. communis L., probably ‘Beurre Giffard’, was discovered in 1954 in an orchard row fence near Cacak, Yugoslavia. Tree vigor, pubescent narrowly lanceolate leaves, large fruit buds, pronounced fruit calyx, large core, and numerous large grit cells are characteristic of P. amygdaliformis. The fruit size and ovate-pyriform shape, however, combined with a high sugar content, are very characteristic of the P. communis parent. In Yugoslavia, the tree appeared highly resistant to pear scab (Venturia pyrina Aderh.), leaf spot [Mycosphaerella sentina (Fekl.) Schroet.] and pear psylla (Psylla pyri L. and P. pyrisuga Foerst.).
Abstract
Natural infections of fire blight following 2 years of epiphytotic conditions caused severe blight damage in the pear cultivar collection at Beltsville, Maryland. Of 522 cultivars rated with the USDA fire blight scoring system, 88% were highly susceptible, 2% were moderately susceptible, 4% were moderately resistant, 5% were highly resistant, and 2% had no blight symptoms. Names of pear cultivars in each resistance class are listed.
Abstract
We rated 86 pear cultivars and selections for their grit content on an inverted scale of 9 to 1. Of these, 64% were rated 6, which we consider to be commercially acceptable, or higher. More than 2000 seedlings from 186 crosses with these cultivars and selections as parents were grouped on the basis of their parental mean grit rating. In each case where the parental mean was 5.0 or higher, the progeny mean was lower than the parental mean. Conversely, in each case where the parental mean was 4.5 or lower, the progeny mean was higher than the parental mean. Where the parental mean was 7.5 or higher, 84% of the seedlings had grit ratings of 6 or higher. Conversely, where the parental mean was 4.0 or lower, only 26% of the seedlings had grit ratings of 6 or higher. We can thus predict the percentage of seedlings in a progeny that can be expected to have a commercially acceptable grit rating. Grit cell development appears to be inherited quantitatively at a minimum of 4 loci. The genes appear to be additive in action rather than dominant.