With the cooperation of six commercial greenhouses (five in Colorado, one out of state), rooted poinsettia cuttings and bedding plant plugs were collected and analyzed for Pythium and Rhizoctonia, two common root rotting pathogens in Colorado greenhouses. Samples of plant, soil, and water debris were taken from four greenhouses, as well as samples of growing media ready for use. These were also analyzed for Pythium and Rhizoctonia. Fungi recovered from the plant, debris, or growing media samples were identified, grown in pure culture, and introduced into susceptible plants (Vinca minor) in pathogenicity studies. Neither pathogen was isolated from the rooted poinsettia cuttings tested. Pythium was not found in any of the plug material or in growing media. Rhizoctonia solani was found in 16% of the plug samples and 7% of the growing media samples tested. Debris from greenhouse floors yielded four species of Pythium as well as Rhizoctonia solani. Isolates of each fungus were able to colonize, but not adversely affect, inoculated plants in pathogenicity studies. It appears that disease causing organisms that have potential to decrease plant quality and growth are already present in the greenhouse. Control of root rotting pathogens can best be carried out by relying heavily on sanitation measures.
Laura Pickett Pottorff and Karen L. Panter
M.A. Chandler, V.A. Fritz, F.L. Pfleger and R.R. Allmaras
Pea root rot is a serious economic threat to pea production in the Great Lakes region. The primary causal organism is Aphanomyces euteiches Drechs., which is responsible for an estimated 10% annual crop loss. A fall oat (Avena sativa) rotation before spring pea planting reduces disease severity. To better understand the beneficial effect of oat on A. euteiches, isolated individual pathogen lifecycle stages of zoospores, mycelium, and oospores were treated in culture with oat extract. Resulting mycelial mats were dried and weighed. Treatment with 90%, 70%, 50%, and 30% oat extract resulted in significant spore germination and mycelial growth of A. euteiches. In the presence of nutrient solution, oat extract concentrations of 90%, 70%, 50%, and 30% significantly enhanced spore germination and mycelial growth of the pathogen. These results demonstrate that the use of oat extract results in dosage dependent germination and growth of A. euteiches.
Chrislyn A. Drake and James F. Hancock
Black root rot is a widespread disease of strawberry (Fragaria×ananassa Duchnesne) that causes the death of feeder roots and the degradation of structural roots. The major causal organisms of black root rot include Rhizoctonia fragariae Husain and W.E. McKeen, Pythiumspp. and Pratylenchuspenetrans(Cobb) Filipjev and Schuurmans Stekhoven. The current method of control for black root rot is methyl-bromide fumigation; however, methyl bromide is scheduled to be phased out in 2005, and its effects are short-lived in matted-row systems. The objectives of the study were to measure levels of tolerance to black root rot in 20 strawberry genotypes and to determine which pathogens were present in the soil. The genotypes were planted in four blocks each of methyl-bromide fumigated and nonfumigated soil, and were evaluated for crown number, number of flowers per crown, yield, and average berry weight over two years. The results showed that all three pathogens were present in the field, and that there was a significant genotype × fumigation interaction for yield and crown number in both years. The cultivars Bounty, Cabot, and Cavendish, all released from the breeding program in Nova Scotia, displayed tolerance to the pathogens that cause BRR. Greenhouse studies were conducted using a subset of the genotypes to determine if any one pathogen causes more damage than others, and to determine if susceptibility to a particular pathogen varies between genotypes.
Jacqueline Joshua and Margaret T. Mmbaga
identification of root rot pathogens and endophytic BCAs. Isolation of root rot pathogens in snap beans ( Phaseolus vulgaris ) was conducted at four different locations, including Tennessee State University (TSU) bean research fields, TSU outreach community
Margaret T. Mmbaga, Lucas M. Mackasmiel and Frank A. Mrema
suppress other fungal pathogens including root rot pathogens ( Mmbaga et al., 2016 ). The objectives of this study were to 1) evaluate the efficiency of six previously selected microbial isolates (B17A, B17B, F13, F16, Y4, and Y14) as biological control
Jim Downer, Ben Faber and John Menge
Mulches can exert positive (disease controlling) or negative (disease enhancing) potential when applied to young avocado (Persea americana) trees. Regulation of root disease in avocado is a complicated process that is affected by host resistance, inoculum density, temperature, soil salinity and soil water potential. There are short-term immediate effects from mulching and subtle long-term effects that regulate disease caused by the root rot pathogen Phytophthora cinnamomi. Short-term effects include increased soil moisture and soil temperature moderation. Long-term effects include increases of: soil mineral nutrients, soil aggregation and drainage; microbial activity; and cellulase enzyme activities. Biological control of Phytophthora in mulched soil is partially regulated by cellulase enzyme activities. This soil enzyme concept of biological control is discussed in regard to the classical Ashburner method of biological control.
Desmond R. Layne, Guido Schnabel, Kerik Cox, Ralph Scorza and Karen Bussey
Armillaria root rot (ARR) of peach caused by the soil-borne basidiomycete fungus Armillaria tabescens is causing premature decline and mortality of peach trees on most southeastern U.S. peach farms. Soil inoculum may be present both in former peach orchard sites and on sites that were once in hardwood forest. The fungus is protected under the bark of dead root pieces and may survive up to 100 years at various depths in the soil profile. No commercially available rootstocks are resistant to ARR. Since 2002, we have embarked on a multipronged strategy to develop control options to combat ARR. First, we have two replicated trials on commercial grower replant sites with a history of ARR. Trial 1 compares four preplant fumigation treatments (none, Telone II, methyl bromide, and Enzone), three rootstocks (Lovell, Halford, and Guardian) and preplant root dips with endomycorrhizal fungi. Trial 2 compares the use of raised beds, root collar excavation and preplant root dips. Both trials examine long-term productivity and tree survival. Second, we are examining the use of systemic fungicide injection into infected trees to protect trees around infection foci. Third, we are trying to develop a genetically modified ARR-resistant rootstock. We have inserted the gene encoding the gastrodia antifungal protein (GAFP—a low molecular weight lectin that binds mannose and chitin) from a Chinese orchid into tobacco (model herbaceous system) and plum (model Prunus system). GAFP has antifungal activity against several basidiomycete root rot pathogens. Pathogenicity tests with transformed tobacco plants show enhanced tolerance to several root rot pathogens when compared to nontransformed plants. Transformed plums are being multiplied for pathogenicity tests.
Roni Cohen*, Yosef Burger and Menahem Edelstein
The use of grafted vegetables as one of the alternatives to soil disinfestation with methyl bromide is increasing in Israel. Watermelon (Citrullus lanatus) and melon (Cucumis melo) plants are grafted mainly onto Cucurbita rootstocks for lessening losses due to soil-borne pathogens. The contribution of the rootstock to the grafted plant's resistance depends on the nature of the disease. In general, damage caused by non-specific root-rot pathogens such as Rhizoctonia solani, Macrophomina phaseolina, Monosporascus cannonballus, and Pythium spp. are effectively reduced by using Cucurbita rootstocks. However, these rootstocks provide only partial protection from vascular diseases such as fusarium wilt, in which case better protection can be achieved by grafting susceptible melons onto monogenic fusarium-resistant melon rootstocks. The performance of the grafted plants depends not only on the rootstock but also on the scion response to pathogens and on the effect of the environment on disease development. The response of grafted and non-grafted melons of different cultivars to sudden wilt disease caused by the fungus Monosporascus cannonballus was evaluated in field trials conducted in the fall and spring growing seasons. Significant differences in disease incidence were found among cultivars, between grafted and non-grafted plants, and between seasons. Grafting reduced plant mortality in the spring and fall experiments but prevention of yield losses was more effective in the spring. More emphasis should be given to finding suitable rootstocks and adjusting agrotechniques for successful commercial cultivation of grafted melons in the fall.
David R. Bryla, Robert G. Linderman and Wei Q. Yang
Fifty-five commercial blueberry (Vaccinium spp.) fields were sampled in northwest Oregon in 2001 to determine the incidence of Phytophthora and Pythium root rot pathogens and identify cultural factors that increase the probability of developing infection. Phytophthora was detected in 24% and Pythium was detected in 85% of the fields sampled. The only species of Phytophthora identified in the study was P. cinnamomi. Root infection by P. cinnamomi was significantly related to cultivar with incidence observed more frequently than expected in ‘Duke’ and ‘Bluecrop’. Both blueberry cultivars are two of the most popular grown in the region, representing 42% of the fields in this survey and ≈46% of the total area planted in Oregon. Two other cultivars found infected by P. cinnamomi were ‘Rubel’ and ‘Briggitta Blue’, together accounting for an additional 24% of the fields surveyed. Phytophthora was not detected in fields planted with ‘Berkeley’, ‘Bluejay’, ‘Bluetta’, ‘Darrow’, ‘Earliblue’, ‘Elliott’, and ‘Powderblue’, each of which represented only 2% to 7% of the fields surveyed. Pythium spp. were identified to genus only, but one or more species of Pythium was found in all 11 cultivars included in the survey. Occurrence of either Phytophthora or Pythium was unrelated to soil type, planting age, or cultural practices such as bed type, cover crop, mulch, irrigation system, fertilizer application, fungicide use, or the source of plant material used in the fields. Overall, most fields with Phytophthora or Pythium remained largely symptomless under good soil drainage conditions and had similar levels of vigor as those without the pathogens.
Kristin L. Getter and Dale W. Rozeboom
substrate moisture ( Mullen and Hagan, 2001 ). Pots containing substrate treatments with a high amount of ATC (like 0P:80A) were much heavier after irrigation than other treatments [i.e., wetter (personal observation)], which may allow root rot pathogens to