Canker Length Research Articles

First Report of Eastern Filbert Blight on Corylus avellana 'Gasaway' and 'VR20-11' Caused by Anisogramma anomala in New Jersey.

Eastern filbert blight (EFB) is a serious disease of European hazelnut, Corylus avellana L., which causes economic losses in Oregon (OR) where 99% of the U.S. crop is produced. The causal organism, Anisogramma anomala (Peck) E. Müller, is native east of the Rocky Mountains where it is found in association with C. americana Marshall. While C. americana is tolerant, EFB causes cankering, branch dieback, and death of C. avellana (3). EFB was first discovered in Washington State in the late 1960s (1). Since then, it has spread throughout the Willamette Valley of OR. In OR, 'Gasaway', an obsolete pollinizer, shows complete resistance to EFB, conferred by a dominant allele at a single locus (4). 'Gasaway' has been widely used in breeding at Oregon State University (OSU) to develop resistant cultivars that are used in most new orchards. In January 2008, cankers containing rows of dark brown elliptical stroma, characteristic of EFB, were first observed on more than 25 trees of 'Gasaway' growing at the Rutgers University research farms in Adelphia and North Brunswick, NJ. At that time, cankers were also found on 18 trees of 'VR20-11' growing on the research farms. 'VR20-11,' an offspring of 'Gasaway' that carries the same resistance gene, was released by OSU for use as a pollinizer for 'Barcelona', an EFB-susceptible but widely grown cultivar in OR. Additional cankers were observed on the New Jersey trees in January 2009 and 2010. To our knowledge, this is the first report of EFB on either cultivar under field conditions. The cankers are smaller than those on susceptible cultivars. Of 61 cankers on 10 trees of 'Gasaway', the average length was 11 cm with a range of 4 to 42 cm. Canker lengths on susceptible trees are typically 20 to 100 cm. The cankers appear otherwise alike with stromata, 2 to 4 × 2 mm, up to 2 mm high; perithecia upright, in the lower part of stroma; asci ellipsoid, 35 to 45 × 9 to 12 μm; and ascospores 8 to 11 × 4 to 5.5 μm, hyaline, smooth, ellipsoid, 2-celled, with the lower cell very short (1 to 1.5 μm long and wide). Genomic DNA was isolated from ascospores excised from cankers of 'Gasaway' and 'VR20-11'. ITS1F and ITS2 primers were used to amplify and sequence the internal transcribed spacer 1 region (ITS1) of the rRNA genes (GenBank Accession Nos. HM565133 and HM565132). BLAST analysis of the 238-bp segments showed 99% homology with a sequence of A. anomala (EU683064). Phylogenetic analysis also confirmed that the two isolates are A. anomala. To test viability, 'Gasaway' cankers were excised and ascospore suspensions (1 × 106 spores ml-1) were applied to 15 trees of susceptible 'Barcelona' in March 2008 following the protocol of Johnson et al. (2). In December 2009, 12 of 15 inoculated trees expressed EFB. 'Gasaway' has shown no signs or symptoms of infection by A. anomala over several decades of exposure in OR, which is believed to have a limited diversity of the fungus due to a single-point introduction. Our findings suggest quarantine efforts must be bolstered to prevent further introductions of A. anomala into the Pacific Northwest to protect the viability of the U.S hazelnut industry.

Effect of QoI Fungicides on Colonization and Sporulation of Monilinia fructicola on Peach Fruit and Blossom Blight Cankers.

Three studies were conducted to examine the curative activity of azoxystrobin, trifloxystrobin, and pyraclostrobin + boscalid against Monilinia fructicola, causal agent of brown rot on peach. In the first study, 'Autumnglo' peach trees were treated with each of the three fungicides both before and after fruit inoculation. In the second study, the effects of fungicide active ingredient, rate/volume, and inoculation timing were examined on inoculated 'Suncrest' peach fruit. Results of these studies showed that sporulating area, on average across all treatments, was reduced by 15.9, 42.4, and 0.4% for azoxystrobin, trifloxystrobin, and pyraclostrobin + boscalid, respectively. In any single treatment, trifloxystrobin provided the greatest benefit with two consecutive sprays, reducing sporulating area by 53 to 60%. In contrast to sporulation activity, the three fungicides exhibited less of an inhibitory effect on fruit colonization. When applied at maximum labeled rates in the various treatments, azoxystrobin, trifloxystrobin, and pyraclostrobin + boscalid reduced colony growth, on average, by 12.3, 7.5, and 7.4%, respectively. Because the pathogen was inoculated into the mesocarp, this low level of activity against colonization may be due to a lack of deeper systemic movement of the fungicides into fruit tissue. In the final study, the three fungicides were examined for their antisporulant activity on blossom blight twig cankers. Unlike results observed on fruit, significant reductions in spore production on cankers were observed for all three fungicides. Azoxystrobin, trifloxystrobin, and pyraclostrobin + boscalid provided 56, 71, and 53% reductions, respectively, in the number of conidia produced per unit canker length. Overall results of these studies indicated that quinone outside inhibitor fungicides, in addition to their known protectant activity, also possess varying levels of curative activity against M. fructicola. In particular, trifloxystrobin demonstrated good antisporulant activity on both fruit infections and cankers.

EFFECT OF NITROGEN LEVELS AND CLIMATE FACTORS ON SUSCEPTIBILITY OF PEACH TREES TO PHOMOPSIS AMYGDALE

The effect of different levels of nitrogen fertilization on the susceptibility to peach constriction canker, caused by Phomopsis amygdali, was studied on a peach orchard of Rich Lady cultivar, from March 2005 to April 2008. Four different nitrogen levels were used: without nitrogen, 50 kg N ha -1 , 100 kg N ha -1 , and 200 kg N ha -1 . Plant inoculations with P. amygdali were carried out in November. The canker length at the inoculation sites was measured between January and March of the following year and used as a susceptibility assessment. Twenty-five leaves per tree were sampled, in June of each year, to obtain an early diagnosis of the nutritional status of the plants. After three years, the plants with the lowest fertilization level showed 3.2% of nitrogen leaf content and those with the highest fertilization showed 3.9%, indicating a small variability in leaf nitrogen content. A positive correlation was found between nitrogen and sulphur and a negative correlation between nitrogen and magnesium. The highest nitrogen fertilization enhanced susceptibility to P. amygdali, which was to a great extent determined by rainfall.

A method for estimating white pine blister rust canker age on limber pine in the central Rocky Mountains

SummaryEpidemiological studies of white pine blister rust on limber pine require a temporal component to explain variations in incidence of infection and mortality. Unfortunately, it is not known how long the pathogen has been present at various sites in the central Rocky Mountains of North America. Canker age, computed from canker length and average expansion rate, can be used to estimate infestation origin and infection frequency. To investigate relationships between canker lengths and canker ages for limber pine, we collected live white pine blister rust branch and stem cankers from three locations in Wyoming and two locations in Colorado. We quantified relationships between various measures of canker length and an estimate of canker age based on dendrochronological analysis. Total branch canker length was strongly, negatively correlated (r = −0.79) with the first year of incomplete, annual ring formation (canker age). Mean longitudinal canker expansion rate was 8.4 cm year−1 for branch and stem cankers where branches distal to the canker were either dead or alive. Annual longitudinal canker expansion, however, was significantly greater on a stem or branch where the portion distal to the canker was alive (11.5 cm year−1) rather than dead (7.1 cm year−1). For branches or stems, proximal expansion rate (i.e., toward or down stem) averaged 4.9 cm year−1. The circumferential canker expansion rate (around branch or stem) was greater for stem cankers (8.3 cm year−1) than for branch cankers (6.2 cm year−1). Additional site and host tree covariates did not improve prediction of canker age. Two simple linear equations were developed to estimate a canker age from total length of a canker with the distal portion either alive or dead. An appropriate sample of canker ages can be used to determine how long a limber pine stand has been infested with white pine blister rust and how frequently infections have occurred.

First Report of Aspergillus Vine Canker on Table Grapes Caused by Aspergillus niger in Europe

In March of 2003, vine cankers and lesions were first observed on vigorous 1-year-old cv. Black Rose vines (Vitis vinifera) in a vineyard in southeastern Sicily, Italy. Since then, the same symptoms have been observed every year up to 2007 throughout the most important table grape production areas of Italy on cvs. Black Rose, Redglobe, and Italia. The disease epidemic has increased during the last 2 years, with incidence of cankered vines as much as 12.5%, and with two to three canker lesions per infected vine. On affected vines, most cankers were found at branch points and on the stems of young shoots grafted in the field. The infections seem to start from growth cracks on rapidly growing 1-year-old shoots or through wounds caused by the removal of lateral shoots. First symptoms usually appeared in late July or August as reddish pinhead-size drops of sap exuded from the infection site on the surface. By October to November, trunks of affected vines were girdled by the canker and prematurely displayed bright fall colors. The trunk appeared slightly larger and spongy where the canker occurred. Internally, cankered tissues were discolored and dead, and black powdery spores were abundant and sometimes visible on the surface. Callous tissue was often associated with the canker. Canker length was variable (average of approximately 8 cm) and either affected a small portion of the shoot's cross section or girdled it completely. Cultures of the fungus isolated from canker lesions or from the black spores within the lesions consistently yielded colonies typical of Aspergillus niger van Tiegh (3) when cultured on potato dextrose agar (PDA) supplemented with 100 μg/ml of chloramphenicol. Pathogenicity tests were carried out on potted plants of cvs. Black Rose, Redglobe, and Italia grown in a greenhouse by inoculating six vigorous, young grapevines of each cultivar. Inoculations were made by placing 8-mm-diameter mycelial plugs taken from actively growing cultures of A. niger on a cut made by removal of a lateral shoot with a sterile razor. Inoculated sites were sealed with Parafilm to prevent dehydration of the inoculum. For a control treatment, an equal number of grapevines was inoculated with only a PDA plug. Three months later, all inoculated grapevines exhibited symptoms similar to those observed in the commercial vineyards while control vines remained symptomless. A. niger was always reisolated on PDA media from all infected vines. A. niger is reported to cause bunch rot complex of grape (1,2) and was previously reported in California causing Aspergillus vine canker (4). Since disease incidence and severity can increase year after year on vines grafted in the field, it is recommended that all cankers be removed by cutting back the vines in the fall. In the following March to April, growers could revisit the affected vines, evaluate shoot growth, and vines with weak shoot growth should be cut back below the canker. Subsequently, the vine can be retrained using a shoot originating from below the canker. To our knowledge, this is the first record of Aspergillus vine canker caused by A. niger on table grapes in Europe.

Response of Butternut Selections to Inoculation with Sirococcus clavigignenti-juglandacearum.

Butternut trees (Juglans cinerea) clonally propagated via grafting from source trees with putative resistance to butternut canker were inoculated monthly with two isolates of Sirococcus clavigignenti-juglandacearum in a field planting in Minnesota. Significant differences in resulting canker length were found among (i) month of inoculation, (ii) accession, and (iii) isolate. Results support the suggestion that canker-free butternut occasionally found growing among butternut severely affected by butternut canker potentially have disease resistance and may have value for genetic conservation and restoration of the species. Developing an effective resistance screening technique will require additional information on the natural infection process and testing under various climatic conditions across the range of butternut.

The use of Verticillium dahliae and Diplodia scrobiculata to induce resistance in Pinus halepensis against Diplodia pinea infection

The pathogenic fungi Verticillium alboatrum and Diplodia scrobiculata were assayed for biological control of Diplodia pinea on Aleppo pine (Pinus halepensis) in Catalonia (north-eastern Spain). Young shoots were pre-treated with inoculations of either V. dahliae or D. scrobiculata, by placing colonized agar plugs on wounds made by removing a single needle fascicle. An inoculation with D. pinea was performed 15 days later. Two months after the shoot inoculations, the canker length on the stems was measured and the percentage of shoot dieback calculated. Verticillium dahliae and D. scrobiculata were found to significantly reduce the canker length of D. pinea (P 0.05) were observed in the mean canker length between the two treatments. Trees pre-inoculated with V. dahliae resulted in 31.12% shoot dieback, while those pre-inoculated with D. scrobiculata resulted in 32.18% shoot dieback, compared with positive controls (42.85%).

Natural and Experimental Host Range of Sirococcus clavigignenti-juglandacearum.

The fungus Sirococcus clavigignenti-juglandacearum causes butternut canker in North America and is not known to kill or seriously affect tree species other than butternut (Juglans cinerea). However, this putative exotic pathogen has been found on branches of black walnut (J. nigra) and heartnut (J. ailantifolia var. cordiformis), raising questions regarding the potential host range of the pathogen and its spread to new areas on tree hosts other than butternut. Results of artificial inoculations of several hardwood species indicated that the pathogen, although not causing lethal cankers, has the ability to colonize and survive in the wood of several genera, including Quercus, Carya, Corylus, Prunus, and Castanea. Several commercially important Persian walnut (J. regia) cultivars tested were moderately or highly susceptible, underscoring the importance of preventing the movement of the pathogen into commercial Juglans plantations in California, where its ability to cause disease and damage trees is unknown. Evidence for the presence of variation in aggressiveness among isolates of S. clavigignenti-juglandacearum was detected based on differences in canker lengths resulting from inoculations with two isolates of the pathogen. In a field test of several previously named butternut cultivars, all were found to be highly susceptible to the disease, calling into question the value of these selections for nut production or future restoration uses.

Description of the elongation of fire blight canker, caused by Erwinia amylovora, in trunks of pear trees

Fire blight is the most destructive pathogen of pears and other pome fruit. The developing fire blight cankers can cause significant damage to a tree. The models described in this study are devoted to the quantitative evaluation of scale of canker and fitted to the situation of artificial inoculations in the beginning of the summer growth phase (season), in 1-year newly developed shoots of perennial trunks. We evaluated the length of the canker at the end of the summer phase based on data collected in the beginning and in the middle of the season. The models based on logistic, Gompertz and Richards functions were applied. The upper asymptotes of the models served for description of the end-of-season fire blight canker length. These asymptotes were estimated for incomplete data of the middle of the season using the suggested computational method. Results suggested that the elongation of the fire blight canker in trunks of pear trees could be described by logistic and Richards models with better precision than by the Gompertz one. The smallest AIC values were observed with the logistic model.

Influence of Ring Nematode Infestation and Calcium, Nitrogen, and Indoleacetic Acid Applications on Peach Susceptibility toPseudomonas syringaepv.syringae

ABSTRACT Two field experiments were conducted to study the effects of added nitrogen, calcium, and indoleacetic acid, in the presence or absence of ring nematodes (Mesocriconema xenoplax), on susceptibility of peach to bacterial canker. When noninfested soil was inoculated with ring nematodes, peach tree susceptibility to bacterial canker infection caused by Pseudomonas syringae pv. syringae was dramatically increased after a period of 2 years. However, no evidence was found that ring nematode infestation increased tree water stress or, in turn, altered plant calcium uptake. Soil fumigation with methyl bromide prior to planting in a commercial orchard significantly reduced both nematode populations and peach tree susceptibility to bacterial canker infection when compared with nonfumigated treatments. In both experiments, tree susceptibility, as measured by canker length following inoculation of stems with P. syringae pv. syringae, was negatively correlated with plant tissue nitrogen content and positively correlated with tissue calcium content. A principal components analysis showed that tissue nitrogen and calcium levels were negatively correlated, and that high-nitrogen, low-calcium tissues were less susceptible to bacterial canker than low-nitrogen, high-calcium tissues. These results indicate that the increased susceptibility of peach to P. syringae pv. syringae under nematode infestation conditions is mediated by both nutritional effects (primarily nitrogen) and nutritional-independent effects, but do not support previous reports of beneficial effects of calcium for reducing bacterial canker.

Interaction Between Nitrogen-Fertilized Peach Trees and Expression ofsyrB, a Gene Involved in Syringomycin Production inPseudomonas syringaepv.syringae

ABSTRACT The in vitro expression of the syrB gene that controls the synthesis of syringomycin, a non-host-specific phytotoxin produced by Pseudomonas syringae pv. syringae van Hall, was studied using aqueous extracts derived from bark tissues collected from nitrogen-fertilized and nonfertilized peach trees. Expression of the syrB gene was quantified as beta- galactosidase activity expressed by P. syringae pv. syringae B3AR-132 containing a syrB::lacZ fusion. Gene expression was significantly less in three of four paired comparisons using extracts derived from fertilized versus nonfertilized trees; however, canker lengths were significantly different in only one of four comparisons. Expression was negatively correlated with plant tissue nitrogen content and positively correlated with a plant carbon/nitrogen ratio. Bark tissue from ring nematodeinfested trees had significantly higher concentrations of total soluble phenolic compounds and carbon/nitrogen ratios than bark samples from trees without nematodes, and canker size was significantly greater in trees growing in ring nematode-infested soil compared with noninfested soil. Nitrogen fertilization significantly decreased the plant carbon/nitrogen ratio, which was positively correlated with the concentration of total soluble phenolic compounds. Canker size developing after bacterial inoculation was positively correlated with higher plant carbon/nitrogen ratios and total soluble phenolic compounds. These results support the hypothesis that one reason why nitrogen fertilization decreases host susceptibility to bacterial canker is by either reducing the amount of plant metabolites that can induce syrB gene expression, or producing or increasing the concentration of compounds that antagonize syrB inducing compounds.

Effects of Phosphonate and Mefenoxam Treatments on Development of Perennial Cankers Caused by Two Phytophthora spp. on Almond

Orchard experiments were conducted to evaluate chemical treatments for management of perennial cankers caused by Phytophthora cactorum and P. citricola on almond (Prunus dulcis) in the San Joaquin Valley of California. Single preventive foliar sprays with phosphonate were tested by wounding and inoculating tree trunks and branches with either pathogen at time intervals from 15 days to 5 months after spraying. One to 3 months after inoculation, resulting cankers were measured. Preventive foliar sprays with phosphonate in the fall or spring suppressed development of cankers for up to 5 months after treatment; mean canker lengths on the trees sprayed with phosphonate before inoculation were 22 to 98% smaller than those on trees that received no phosphonate. In subsequent tests, preventive chemigation with phosphonate in spring or summer also inhibited canker expansion. A curative topical treatment with either fosetyl-Al or mefenoxam on cankers caused by P. cactorum or P. citricola reduced canker expansion by 36 to 88%. Neither preventive nor therapeutic treatments eradicated the pathogens from the diseased tissues, but disease ratings made 1 year after the treatments indicated extended disease suppression. Phosphonate and mefenoxam treatments can effectively manage almond scion cankers caused by P. cactorum or P. citricola.

First Report of Cytospora Canker Caused by Cytospora chrysosperma on White Poplar in Israel.

A new disease, causing death of mature white poplar trees (Populus alba L.), was observed in Hulla Valley in northern Israel in the summer of 2002. The affected branches turned yellowish brown, and the inner bark turned black. The bark dried out and separated from the underlying wood. Later, copious, dark pycnidia developed on the dead bark. The pycnidia had a diameter of 650 µm (n = 50), ranging 600 to 800 µm. Under moist conditions, spore masses oozed out in long, reddish brown, coiled tendrils. The spores were hyaline, one-celled, and slightly curved, 1.1 × 5.5 µm (5.0 to 6.0 µm) (n = 100), and somewhat smaller than those reported by Schreiner (1). A herbarium specimen was deposited at the U.S. National Fungus Collections (BPI 843390). Isolations made from affected branches yielded colonies of Cytospora chrysosperma (Pers.:Fr.)Fr. with a whitish orange mycelium that turned dark green 11 days later. Its growth rate on potato dextrose agar at 25°C was 7.1 mm per day. Exposure to daylight induced pycnidial development after 3 to 4 weeks. Inoculation of eight 1-year-old seedlings of white poplar and willow (Salix acmophylla Boiss) proved the pathogenicity of several isolates of C. chrysosperma. The average canker length at 28 days after inoculation was 28.0 and 14.5 cm on white poplar and willow, respectively, indicating the higher susceptibility of P. alba. No cankers developed on the control seedlings. Reisolations from inoculated plants yielded C. chrysosperma. To our knowledge, this is the first report of Cytospora canker on white poplar in Israel. Reference: (1) E. J. Schreiner. Am. J. Bot.18:1, 1931.

Prediction of Long-Term Canker Disease Damage from the Responses of Juvenile Poplar Clones to Inoculation with Septoria musiva.

Stem cankers caused by Septoria musiva severely limit production of susceptible hybrid poplars in eastern North America. A field experiment was conducted to determine whether short-term responses of poplar stems to inoculation with S. musiva were predictive of long-term canker disease damage. Stems of 27 poplar clones were inoculated during their first season of growth by removing the fourth or fifth fully expanded leaf and placing an agar plug colonized by an aggressive isolate of S. musiva over the resulting wound. Four months after inoculation, incidence of cankers, canker length, and percent of stem circumference affected (girdle) were recorded. Clones varied greatly in canker incidence (12 to 98%), mean canker length (10 to 53 mm), and mean girdle (14 to 94%). Logistic regression analysis was used to compare these inoculation responses with canker disease damage categories assigned on the basis of information from longer-term field studies. Incidence, canker length, and girdle data were all informative, but girdle data from this field experiment correctly predicted assigned canker disease damage categories most frequently (24 of 27 clones). Responses of 15 of these clones also were evaluated in a similar greenhouse experiment. Although responses usually were predictive of long-term damage categories, the probability of correct prediction was lower than that obtained in the field experiment for most clones. These results demonstrate the feasibility and potential benefit of screening juvenile poplar clones for responses to inoculation with S. musiva before undertaking field trials for productivity and subsequent release to growers.

Responses of conifer species of the Great Lakes region of North America to inoculation with the pitch canker pathogen Fusarium circinatum

SummaryFive conifer species grown in the Great Lakes region of North America were examined for their susceptibility to Fusarium circinatum, (syns. Fusarium subglutinans f. sp. pini and F. moniliforme var. subglutinans), the causal agent of pitch canker. Three‐year‐old (3‐0) seedlings of red (Pinus resinosa), jack (P. banksiana) eastern white (P. strobus), Scots (P. sylvestris) and Austrian (P. nigra) pine were planted in 4 l pots in a greenhouse at Auburn University in November 1998. In April and June 1999, seedlings were inoculated by removing a needle fascicle approximately 5 cm from the terminal bud and placing a drop containing F. circinatum conidia on the wound. Resin production, canker length and seedling mortality were recorded 12 weeks later. Jack, Scots and eastern white pine were the most susceptible with Austrian and red pine more resistant to the fungus. F. circinatum was re‐isolated from 37% to 96% of inoculated seedlings. The susceptibility of jack, Scots and eastern white pine indicates a potential risk to these important species of the region if F. circinatum were to be introduced into the area.

The Role of Autumn Infections in the Progression of Fire Blight Symptoms in Perennial Pear Branches.

The role of autumn infections in the progression of fire blight (caused by Erwinia amylovora) symptoms in perennial pear branches was studied in orchard-grown trees in Israel. The extent of symptom progression and the final length of fire blight cankers in perennial branches were variably affected by the vigor of the trees and the season of infection. Following spring infections, when all trees supported active shoot growth, fire blight symptoms progressed more rapidly and to longer distances in trees that exhibited high vigor (i.e., with numerous annual shoots on most terminal branches) than in low-vigor trees (i.e., few or no annual shoots on terminal branches). Irrespective of the vigor of the trees, the progression of fire blight symptoms in perennial branches ceased between mid-May and mid-July, and only a small proportion (0 to 14.2%) of the infections had invaded main limbs or trunks of trees. Progression of fire blight symptoms following autumn infections was related to the preceding summer (August to No-vember) shoot regrowth: in trees in which the shoots did not restore their growth in the summer, the rate of symptom progression in perennial branches was higher in trees with a low vigor than in those with a high vigor, whereas for those with summer regrowth the relationship between rates of symptom expression was reversed. Irrespective of the vigor group and of whether there was summer regrowth, symptoms in perennial branches continued to progress through the winter until the following spring. Most of the autumn infections (50 to 78.5%) that developed in susceptible trees had invaded main limbs or trunks of trees. The results of this study indicate that factors related to host phenology and physiology, rather than factors related to environmental influences (such as temperature), govern the extent, rate, and duration of fire blight progression in perennial pear branches. Furthermore, it turned out that autumn infections play a substantial role in fire blight epidemiology in Israel.

Seasonal changes in susceptibility of Quercus suber to Botryosphaeria stevensii and Phytophthora cinnamomi

Monthly inoculations of both intact plants and excised shoots of Quercus suber with the pathogenic species Botryosphaeria stevensii and Phytophthora cinnamomi were performed to investigate seasonal changes in susceptibility of this forest tree species in relation to environmental parameters and plant water status. Infection symptoms were mainly detected on seedlings inoculated from spring to autumn (April through October) with either pathogen. Mean canker sizes also showed a seasonal pattern, the higher values being recorded in the same period as above. Lesion lengths were significantly (P < 0·001) related to environmental minimum temperature. Mean daily minimum temperatures within the range of 5–12°C clearly inhibited lesion development of P. cinnamomi, whereas B. stevensii showed a less pronounced decrease in canker expansion at the same temperature range. In excised shoots of Q. suber inoculated monthly with B. stevensii, a negative linear relationship was found between the studied range of plant relative water content (81–91%) and canker length. In contrast, the lesions caused by P. cinnamomi were not significantly (P = 0·32) related to any seasonal change in water content. Some control measures for the diseases caused by both pathogens are discussed on the basis of the seasonal changes in host susceptibility observed in this study.

Suppression of Phytophthora Root and Crown Rot on Pepper Plants Treated with Acibenzolar-S-Methyl.

The fungicide mefenoxam is registered for the control of Phytophthora blight of peppers caused by Phytophthora capsici. Isolates of the pathogen that are insensitive to mefenoxam, however, have been detected in some locations. Consequently, alternative methods are needed to control Phytophthora blight of peppers. Acibenzolar-S-methyl (ABM, Actigard) is a chemical activator of plant disease resistance that has potential for the management of Phytophthora blight of peppers. The effect of foliar applications of ABM on the development of root and crown rot on pepper plants grown in the greenhouse and inoculated with Phytophthora capsici or in soil naturally infested with the pathogen was evaluated. Inhibition of stem canker development on pepper cvs. Bell Tower and AZ9 after four treatments with ABM (75 μg/ml) was significantly greater than on plants receiving a single application of the chemical. Stem canker length on Bell Tower or AZ9 peppers was inhibited by 93.2 to 97.2% and 87.4 to 92.4% when plants were inoculated with P. capsici at 1 or 5 weeks, respectively, after the fourth application of ABM. Survival of chile pepper plants grown in field soil naturally infested with P. capsici was significantly increased by three foliar applications of ABM (75 μg/ml) compared with nontreated plants in all three trials when pots were watered daily and in two of three trials when pots were flooded for 48 h every 2 weeks. When soil was flooded every 2 weeks to establish conditions highly favorable for disease development, plants treated once with mefenoxam (100 μg/ml) survived significantly longer than those treated with ABM. On the other hand, when water was provided daily without periodic flooding to establish conditions less favorable for disease development, plant survival between the two chemicals was not different in two of three trials. Length of survival among chile pepper plants treated twice with 25, 50, or 75 μg/ml of ABM and grown in soil infested with P. capsici was not different. This work indicates that ABM could be an important management tool for Phytophthora root and crown rot on pepper plants.

Preliminary evaluation of nine fungicides for control ofPhytophthora cactorum andP. citrophthora associated with crown rot in peach trees

Excised twig assay and excised stem inoculation were used to evaluate nine fungicides (metalaxyl, fosetyl-Al, copper hydroxide, copper sulfate, copper oxychloride, captan, quintozene, propamocarb and chlorothalonil) againstPhytophthora cactorum andP. citrophthora associated with crown rot in peach trees. Segments were soaked in fungicide solutions at different concentrations and then inserted vertically intoP. cactorum orP. citrophthora cultures growing on cornmeal agar plus antibiotics, or inoculated by inserting a mycelium-bearing agar plug directly into the cambium. Following incubation, the bark was scraped off and length of necrosis was measured. Metalaxyl was the only fungicide that inhibited canker development on segments at the manufacturer-recommended concentration. Fosetyl-Al, captan, copper hydroxide and copper sulfate inhibited canker development at 3, 4, 4 and 8 gl-1, respectively. The other fungicides did not affect canker length significantly compared with non-treated twigs, with the exception of propamocarb, which reduced the development ofP. cactorum on excised stems. The tested methods enabled rapid and effective evaluation of a large number of chemicals to prevent crown rot diseases caused byPhytophthora in the laboratory.

First Report of Aspergillus Vine Canker of Table Grapes Caused by Aspergillus niger.

A vine canker was first observed in the San Joaquin Valley, CA, in fall 1989, on exceptionally vigorous 1-year-old cv. Redglobe vines (Vitis vinifera) when vines were trained up the stakes. Since 1989, the same canker symptoms have been observed in Tulare, Kern, Fresno, and Riverside (Coachella Valley, CA) counties on cv. Redglobe, Crimson Seedless, Chardonnay, and Grenache vines. In affected vineyards, the disease resulted in the retraining of 2 to 6.1% of vines the following spring, using a shoot originating from below the canker. In a sample of 54 infected vines collected in 1997, 65% of cankers were found at the branching (crotch) of the vine, 24% along the shoot, or both (11%). All infections started through wounds caused by removing lateral shoots or leaves when the vine was topped to form cordons or possibly through growth cracks that occur on rapidly growing 1-year-old shoots. The first symptoms usually appear in August as red pinhead-size drops of sap on the surface of discolored tissue. By October to November, the canopies of vines girdled by the canker prematurely display fall colors and are very distinct from healthy vines. The trunk is slightly swollen and spongy where the canker occurs. Internal canker tissue is discolored and dead. Black spores are abundant within the canker, on the surface of the canker, or both. Callous tissue is often associated with the canker as the vine attempts to repair the damage with new tissue. Canker length can range from 3.5 to 26.5 cm (average 7.0 cm) and can affect the shoot's cross section from 0.4 cm to completely girdling the shoot (up to 9.0 cm in circumference). Isolations from cankers or black sporulation inside the canker on acidified potato dextrose agar (APDA) consistently yielded Aspergillus niger van Tiegh. Six well-matured current-season canes of cv. Redglobe in an experimental vineyard at Kearney Agricultural Center were inoculated by inserting a 7-mm plug of mycelium from actively growing cultures on APDA in a cut made with a 7-mm cork borer or by brushing spores of the culture over the surface of six canes wounded with a sterile razor. Six canes were inoculated with a 7-mm plug of APDA and used as noninoculated controls. Inoculated sites were sealed with Parafilm to avoid dehydration. Inoculation of grapevines with A. niger resulted in cankers similar to those observed in commercial vineyards 5 months after inoculation. Cankers ranged from 2.4 to 4.2 cm for mycelial-plug inoculation (100% of canes infected) and 2.3 to 7.3 cm for spore-brushing inoculation (67% infected). Noninoculated control canes were not infected. In another experiment, inoculation of 10 canes each with A. niger on 17 May, 10 June, 2 July, 21 July, and 16 August resulted in 50, 60, 90, 90, and 100% canker formation, respectively, 5 to 8 months after inoculation, suggesting summer inoculations were more effective than spring inoculations. Reisolation from infected canes on APDA revealed A. niger. Aspergillus species in section Nigri have been reported to be among the pathogens involved in the bunch rot complex (1,2), but to our knowledge, this is the first report of A. niger causing a serious canker of vigorously growing grape vines.