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T.G. Beckman, P.L. Pusey and P.F. Bertrand

Peach tree fungal gummosis caused by Botryosphaeria dothidea [(Moug.:Fr.) Cos & de Not.] is widespread throughout the southeastern United States. Until recently, its economic impact on peach [Prunus persica (L.) Batsch] has been impossible to estimate, since no effective controls were known. Significant, though not total, suppression of gummosis on `Summergold' peach trees was achieved with an intensive 5-year spray program with captafol. Captan was far less effective than captafol. Both trunk diameter and fruit yield were negatively correlated with disease severity. After eight growing seasons, trees treated with captafol were 18% larger than the untreated trees. Yield of mature captafol-treated trees was 40% to 60% high er than that of untreated ones. Following termination of the spray program after 5 years, disease severity gradually increased on both captafol- and captan-treated trees. However, through eight growing seasons, disease severity was significantly lower on captafol-treated trees. This study demonstrates that peach tree fungal gummosis significantly depresses tree growth and fruit yield on susceptible peach cultivars.

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Lusike Wasilwa*, Viincent M. Kega and Richelle A. Stafne

Citrus is one of the most important fruits in Kenya because of it's nutritional value. It is ranked as the highest source of income from tree crops in Kwale district of the coastal region. The average farmer has 0.25 to 4 acres of citrus and earns between 1 to 2 U.S. cents per fruit based on quality. Citrus gummosis (P. citrophthora and P. nicotianae) is of economic importance on citrus in Kenya. P. citrophthora prefers cooler conditions in the highlands and P. nicotianae favours the warm conditions of coastal areas. Several IPM options are being used to control this disease in Kenya. Research to select suitable rootstocks, determine the period when disease development is greatest and the distribution of infected trees in Kwale districts is currently being conducted. Gummosis has also been reported to occur in farmers fields around Marigat (Perkerra) and surrounding areas however the data is scanty and a detailed survey is underway. Gummosis disease incidence in Marigat was first reported in 1989/1990. Since then symptomatic trees have been treated with fungicides e.g. Ridomil. A citrus mother block consisting of 241 trees, established at Perkerra in 1984 now only supports 112 trees or 47% of the original trees. From the 112 trees, only 21 trees are classified as healthy trees. More extensive research needs to be conducted to show the impact of this disease on the small scale farmer and develop better methods of control.

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Charles Arthur Powell

Rio Grande Gummosis (RGG) is a serious disorder of grapefruit in the Caribbean Basin ( Childs, 1978 ). It has also been referred to as Florida gummosis and ferment gum disease (in California) ( Calavan, 1961 ). The disorder has distinct symptoms

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K.C. Taylor

The absence of red color in a streaked “Bleaching” pattern is periodically noted on late-season peaches in middle Georgia. The streaked pattern led to a hypothesis that accumulation of pesticides in the stem end of the fruit prevented anthocyanin formation. However, analysis of pesticide residues on affected and unaffected peel suggested this was unlikely. We observed that trees affected by fungal gummosis (caused by Botrysphearia dithodia) were most often affected by the “bleaching” phenomenon and that `Summer Gold', the most fungal gummosis–susceptible variety, had the greatest incidence of the disorder. In a preliminary trial, we tested the hypothesis that fungal gummosis mediates “bleaching” by interfering with anthocyanin color formation in the peel of developing fruit. Tree gum/resin and pesticides were tested for their effect on peel color development. The gum/chemical preparations were dripped onto fruit prior to anthocyanin or red pigment formation in peach peel. After the anthocyanescent period, fruit were observed for bleaching. The gum mediated a negative effect by sulfur, captan, and carbaryl in peel color formation in peach. Fenbuconizole and phosmet had a less negative effect on color formation, although the effect was noticeable. The gum alone, propiconizole, and chlorothalonil did little to effect on peel color formation.

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K.O. Britton, F.F. Hendrix, P.L. Pusey, W.R. Okie, C.C. Reilly and J.W. Daniell

Two field experiments were conducted to assess peach (Prurus persica L.) cultivar susceptibility to the three Botryosphaeria spp. that cause peach tree fungal gummosis. Inoculated trees were evaluated for disease severity by rating gum exudation, vascular discoloration, and fungal colonization. Each severity measurement yielded a different rank ordering of cultivars for susceptibility. However, in a greenhouse study, these same measurements gave consistent rankings for aggressiveness of the fungal species on `Blake'. Despite large differences in disease severity in the greenhouse study, none of the severity measures were correlated with tree growth after inoculation. The only factor significantly correlated with growth rate of the trees after inoculation was growth rate before inoculation.

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Y.C. Li, A.K. Alva, D.V. Calvert, R.M. Sonoda, R.R. Pelosi and D.J. Banks

The objective of this study was to evaluate the role of chloride (Cl-) and other anions in soil water on the incidence and severity of Rio Grande Gummosis (RGG) in grapefruit (Citrus paradisi MacFadyen) trees. White `Marsh' grapefruit trees on sour orange (C. aurantium Lush.) rootstock were grown on two-row raised beds. Due to differential rates of K application (as KCl), the trees received various rates of Cl- ranging from 0 to 156 kg·ha-1 per year. Soil water was sampled using suction lysimeters at 120- and 180-cm depths. The severity of RGG was evaluated 18 months following the application of various rates of KCl. The concentration of Cl- in soil water samples varied from 16.2 to 617 mg·L-1, with mean concentrations across all treatments of 160.7 and 188.4 mg·L-1 for 120- and 180-cm depth water samples, respectively. The concentrations of Cl- and other anions (\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NO}_{3}^{-}\) \end{document}, \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{PO}_{4}^{3-}\) \end{document}, \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{SO}_{4}^{2-}\) \end{document}) in soil water at either 120- or 180-cm depth were poorly related to the incidence or severity of RGG (R = -0.10 to -0.28). Chloride concentrations in the soil water ranged from 20 to 617 mg·L-1 near the RGG-free trees, and from 16.7 to 310 mg·L-1 near the trees that were affected by RGG. This suggests that under the conditions of this study, neither the incidence nor the severity of RGG was influenced by high concentrations of Cl- in the soil water.

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Don C. Elfving, Gregory A. Lang and Dwayne B. Visser

Prohexadione-Ca (P-Ca) and ethephon (ETH) were evaluated as potential inhibitors of growth and promoters of early flowering for high density orchard management of sweet cherry (Prunus avium L.) trees on vigorous rootstocks. Single applications (P-Ca at 125 to 250 mg·L-1 active ingredient (a.i.) or ETH at 175 to 200 mg·L-1 a.i.) to young, nonfruiting sweet cherry trees produced short-term, generally transient reductions in terminal shoot elongation, and did not stimulate flower bud formation. Tank-mix applications (P-Ca + ETH) usually produced a stronger, possibly synergistic, reduction in shoot growth rate. Single tank-mix applications either increased subsequent flower bud density on previous season shoots or had no effect; when a second application was made three weeks later to the same trees, subsequent flower bud density on previous season shoots and spurs on older wood increased ≈3-fold over untreated trees. Yield efficiency (g·cm2 trunk cross-sectional area) also increased nearly 3-fold. Chemical names used: (2-chloroethyl) phosphonic acid (ethephon); calcium 3-oxido-4-propionyl-5-oxo-3-cyclohexene carboxylate (prohexadione-Ca); polyoxyethylene polypropoxypropanol, dihydroxypropane, 2-butoxyethanol (Regulaid); aliphatic polycarboxylate, calcium (Tri-Fol).

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Elzbieta Z. Krzesinska and Anita Nina Miller Azarenko

Au excised twig assay was developed to evaluate cherry rootstocks (hybrids of Prunus avium L.; P. canescens Bois.; P. cerasus L.; P. fruticosa Pall.; P. mahaleb L.; P. pseudocerasus Lindl.) for their tolerance to Pseudomonas syingae pv. syringae van Hall. Twigs of `Napoleon', `Corum', and F12/1 in 1988 and 10 more rootstocks in 1989 were inoculated with water, one avirulent (K4), or one of three virulent strains (W4N54, AP1, and B-15) of bacteria at 105, 106, and 107 colony forming units (cfu)/ml in 1988 and with 107 cfu/ml in 1989. Evaluation of browning and gummosis at the inoculation site after incubation for 4 weeks at 15C and high relative humidity revealed no gummosis or browning on twigs inoculated with water or the avirulent strain. The amount of browning and gummosis induced by concentrations of 106 and 107 cfu/ml of the virulent strains was, in general, not different within genotypes. `Napoleon' and `Corum' had significantly higher browning and gummosis ratings in 1989 than F12/1 and the 10 rootstock selections, most of which did not differ from F12/1. Rootstocks Gisela (Gi.) 172-9 and Gi. 169-15 had higher incision browning than F12/1 in some instances.

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Elzbieta Krzesinska and Anita Nina Miller

An excised twig assay was developed to evaluate cherry geno-types for their tolerance to Pseudomonas syringae pv. syringae. One-year-old wood was collected at monthly intervals from October until January of `Royal Ann', `Corum', and a number of cherry rootstock. The rootstock included; F/12–1 and Giessen (GI) and M × M selections. A 2-cm incision (“^”-shaped flap) was made on each twig. A 20-μl droplet of inoculum or water was placed onto each incision. The inoculum was prepared with one avirulent (K4) and three virulent strains (W4N54, AP2, B15) concentrations (105, 106, or 107 cfu). Inoculated twigs were placed in test tubes and incubated at 15C in high relative humidity for 3 weeks. After incubation, twigs were evaluated for gummosis production (0–3, 0 = no gummosis), incision browning (1–4, 1 = yellow pith), and callus production (0–1, 0 = no callus). The concentration of bacterial suspension had no effect on symptom development. No gummosis or browning was observed on twigs inoculated with water or the avirulent strain. Based on the gummosis and browning ratings, rootstock M × M 2, M × M 39, M × M 60, GI 148-1, GI 154-2, and GI 154-4 were found to be resistant to these three strains of P. syringae in this assay. Root-stock F 12-1, GI 169–15, GI 172–9, and GI 173-9 were found to be tolerant.

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D.E. Deyton, D.W. Lockwood and S. Ventataraman

A factorial arrangement of four replications of ethephon (0, 25, 50, 100, or 150 mg·liter-1) and GA3 (0, 25, or 50 mg·liter-1) treatments in a Randomized Complete Block Design were applied to `Redhaven' peach trees in mid-September. Each tree received the same treatment in 1987-1990. Development of flower buds (after endodormancy completion) was significantly delayed by GA3 and ethephon. The date of 50% bloom was significantly delayed by GA3 (approximately 1 day) and by ethephon (4.7 days with 150 mg·liter-1 treatment). Increasing the concentration of each chemical resulted in more delay of bloom. There was no interaction of the effects of the two chemicals on bloom date. Application of 50 mg·liter-1 GA3 plus 150 mg·liter-1 ethephon caused the greatest bloom delay (6.5 days compared to untreated trees). Gummosis on scaffolds was evident in the fall and following spring on trees treated with the 2 highest rates of ethephon. During the summer and following fall, little gummosis was evident. By September 1991, evidence of gummosis was insignificant and no tree mortality occurred.