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J. Pinochet, A. Camprubí, C. Calvet, C. Fernández, and R. Rodríguez Kábana

The beneficial effects of early mycorrhizal inoculation with two arbuscular fungi, Glomus mosseae (Nicol. and Gerd.) Gerd. and Trappe and Glomus intraradices Schenck and Smith, were evaluated on Myrobalan 29 C (Prunus cerasifera × Prunus munsoniana Wight and Edr.) plum rootstock in soil infested or noninfested with the root lesion nematode Pratylenchus vulnus Allen and Jensen under microplot conditions. During this two year study, mycorrhizal colonization did not affect the number of nematodes per gram of root in plants infected with P. vulnus. In contrast, P. vulnus significantly decreased the percentage of mycorrhizal root colonization. Most elements were within sufficiency levels for plum by foliar analysis, although low P and deficient Fe and Cu levels were detected in P. vulnus treatments. Early mycorrhizal inoculation with G. mosseae favored plant growth after 20 months, but in soils infested by P. vulnus, only G. intraradices increased the tolerance of Myrobalan 29 C plum rootstock to damaging nematode levels by stimulating plant nutrition and vegetative growth.

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Esmaeil Fallahi, Brenda R. Simons, and Max W. Williams

Effects of hydrogen cyanamide and Wilthin on blossom thinning and the consequences of thinning on fruit set, yield and fruit quality of `Rome Beauty' was studied. A full bloom application of hydrogen cyanamide at the rate of 0.25% (Dormex formulation) or 0.25% of Wilthin both followed by a fruit thinning by Sevin + NAA effectively thinned mature trees of `Rome Beauty' and had a similar effect on fruit set, yield and fruit quality. The effects of these two chemicals at these rates on several aspects of fruit set, yield and quality were similar to the effects of Elgetol. Hydrogen cyanamide, Elgetol and 0.25% Wilthin at full bloom resulted in a higher percentage of single fruit set, thus, less labor for hand thinning. Application of 0.37% Wilthin at 20% bloom or at full bloom resulted in larger fruit size, but induced fruit russetting. Soluble solids of fruit from trees with Elgetol, 0.37% Wilthin at 20% bloom or at full bloom were higher than fruit from other treatments. Hydrogen cyanamide at 0.50% resulted in a satisfactory level of blossom thinning in `Friar' plums.

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B.A. Murdock and N.H. Ferguson

The influence of fall applications of ethephon and gibberellic acid to produce bloom delay in `Wade' plum were carried out in 1987-89. Single saturation applications of ethephon at either 125 and 250 mg·liter-1 with 50 mg·liter-1 gibberellic acid (GA3) at first sign of leaf fall in October of 1987 produced bloom delay of 6-17 days the following spring. No difference in bloom delay was seen between the two treatments. Gummosis and death of some lateral shoots were observed in both treatments but was more severe at the higher ethephon concentration. There was no adverse effect on flower bud opening in either treatment, In 1988 untreated trees suffered 100% fruit loss because of frost injury while both treatments allowed for excellent yields as a result of frost avoidance. In the fall-spring of 1988-89 the experiment was repeated on the same trees; a 5-8 day delay in bloom was observed in both treatments but a late freeze destroyed all fruit on both treated and untreated trees.

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Maria-Jose Rubio-Cabetas, Jean-Claude Minot, Roger Voisin, D. Esmenjaud, Georges Salesses, and Abel Bonnet

In `Myrobalan' plum (Prunus cerasifera Ehr.), Ma1 and Ma2 are single major dominant genes that control the resistance to the predominant root-knot nematode (RKN) species Meloidogyne arenaria (Neal) Chitwood, M. incognita (Kofoid & White) Chitwood, and M. javanica (Treub). These genes were evaluated for activity to the northern RKN M. hapla Chitwood and the tropical RKN M. mayaguensis Rammah & Hirschmann, neither of which is controlled by the Mi gene from tomato. This study was conducted under greenhouse conditions using a resistance screening based on high and durable inoculum pressure by the nematodes. Tests were conducted simultaneously for: M. arenaria (as a reference for the Ma genes); M. hapla and M. mayaguensis from crosses segregating for either Ma2 alone or Ma2 and Ma1 and involving the resistant parental clones P.2175 (heterozygous for Ma1) and P.1079 (homozygous for Ma2); and the host parental clone P.2646 (recessive for both Ma genes). Each parental clone and each individual of the segregating progenies reacted in a similar way to M. arenaria and M. mayaguensis, indicating that the Ma genes also control resistance to M. mayaguensis. By contrast, all parental clones and progenies were completely resistant to M. hapla, and, despite high inoculum pressure, no effect of the Ma genes on this species could be established.

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Noboru Muramatsu, Naoki Sakurai, Ryoichi Yamamoto, and Donald J. Nevins

A nondestructive, acoustic method was applied to evaluate firmness of nectarines (Prunus persica Batch.), apricots (Prunus mume Sieb. et Succ.), plums (Prunus salicina Lindl.), and tomatoes (Lycopersicon esculentum Mill. `Beiju'). Sound with frequencies from 200 to 2000 Hz, generated by a miniature speaker attached to the fruit surface, was received by a small microphone attached to the opposite side. The signal was monitored by an oscilloscope. Sound frequency did not change during propagation in the fruit. However, as the microphone was moved along the circumference of the fruit, a phase shift in the received signal was observed. When the distance the microphone was displaced along the surface of the fruit corresponded to a shift of exactly one wavelength, the sound wavelength propagated within the fruit could be determined. The number of sound waves within the fruit over half its circumference was calculated as a function of this distance. Mature fruit propagated shorter wavelengths and consequently more sound waves than immature fruit, indicating that the sound velocity in the mature fruit was lower than in immature fruit. This relatively simple method for measuring lower frequency suggests that the sound velocity propagated through fruit can be determined without measuring the absolute velocity.

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B.L. Topp, W.B. Sherman, R.E. Stall, G.V. Minsavage, and C.J. Wilcox

Four greenhouse leaf inoculation methods for screening Japanese plum (Prunus salicina L. and hybrids) for resistance to Xanthomonas campestris pv. pruni (Smith) Dye were compared for repeatability, ability to differentiate among plant genotype responses, and correlations with field ratings. Clonally propagated trees were inoculated artificially in a greenhouse by immersing leaves in 2.5 × 108 cfu/ml inoculum (DIP), rubbing the adaxial side of leaves with a slurry of 2.5 × 108 cfu/ml inoculum and Carborundum powder (CARB), infiltrating leaves with 5 × 105 cfu/ml inoculum using a needle-less syringe (INFS), and infiltrating with 5 × 106 cfu/ml inoculum (INF6). No greenhouse method was superior in all assessment categories. The CARB method was most repeatable (t = 0.78) but had a low Spearman's correlation (rs = 0.29), indicating that greenhouse rankings did not correspond closely with field rankings. The INF6 method was unsuitable because it did not differentiate between plant genotypes. The DIP method appeared most suitable, having moderate repeatability (t = 0.46) for four observations per leaf and moderate Spearman's correlation with field performance (rs = 0.56). The INF5 method may be appropriate for identifying bacterial spot resistance that is associated with resistance in the leaf mesophyll.

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A.S. Devyatov

Root systems of 6-year-old trees of plum cultivar Edinburg on seedling rootstocks of Pr. divaricata(I), Pr. tomentosa (II), and clonal rootstock VVA (Pr. tomentosa × Pr. divaricata) (III) were studied with trench-monolith method, at a depth of 100 cm. Sodpodzolic loess soil had a A horizon (0–22 mm) with 2% humus. The area of cross-section on tree trunks on I, II, and III was 72, 44, and 18 cm2, respectively; yield efficiency was 0.22, 0.30, and 0.33 kg·cm–2, respectively. Specific length of scaffold roots of I and III was equal, but roots of I were deeper in the bed. Specific mass of scaffold roots of III was 35% less than I, but specific mass of fibrous roots of III was 84% more. All root indexes of II were many times smaller than I and III. Nearly 40% of total length of scaffold roots of all rootstocks was at the distance of 0.5–1.0 m from the trunk. Fibrous roots of II and III concentrated near the trunk. Specific mass of these roots of I at a distance of 2.0–2.5 m from the tree was twice those near the trunk.

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T.G. Beckman and P.L. Pusey

Armillaria root rot is the second leading cause of peach tree mortality (after peach tree short life) in the southeastern United States. Currently, there are no commercially available rootstocks for peach with proven resistance to this pathogen in the United States. Since 1983, we have been screening rootstock candidates for resistance to Armillaria utilizing naturally infected field sites. Inoculation of peach [Prunus persica (L.) Batsch], plum (P. cerasifera J.F. Ehrh., P. munsoniana F.W. Wight & Hedr., P. salicina Lindl. or P. angustifolia Marsh.) × peach and plum × plum hybrid rootstocks with infected plant tissue (such as acorns, Quercus sp.) prior to planting has provided a significantly increased infection and mortality of candidate rootstock lines in comparison with sole reliance on natural inoculum on an infested site.

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Maria L. Badenes, Jose Martínez-Calvo, Helena Gómez, and Elena Zuriaga

, and self-compatibility, and are resistant to sharka Plum pox virus (PPV), a serious limiting factor for apricot fruit production in affected areas. Their fruits have excellent organoleptic characteristics and are notably larger than the traditional

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Dario J. Chavez, Thomas G. Beckman, and José X. Chaparro

Taxonomic conflicts in Prunus were reported by Waugh (1901) and Hedrick (1911) . Waugh (1901) stated that “plums grow pretty much as they please, and the botanist has to take them as he finds them.” He recognized that new classifications