's survival in cultivation is Daphne Sudden Death Syndrome (DSDS), a disease incited by the fungal root pathogen Thielaviopsis basicola (Berk. & Br.) Ferraris (syn. Chalara elegans Nag Raj et Kendrick) ( Noshad et al., 2006 ). This disease kills plants
David Noshad, Andrew Riseman and Zamir Punja
One of the main difficulties in controlling root diseases biologically has been the inability of biocontrol agents to establish and persist in the rhizosphere. The inability of biocontrol agents to establish and persist is often attributed to competition from indigenous microorganisms for space and nutrients and to fluctuations in environmental conditions. The use of biocontrol agents over the entire geographic range of a crop also has been limited by differences in environmental and edaphic conditions from field to field and region to region. An advantage of hydroponic crop production in greenhouses is that environmental conditions such as temperature, moisture, pH, and growth medium can be consistently controlled in a house and from site to site. An additional advantage of many hydroponic systems is that they are virtually sterile upon planting. This initial period of virtual sterility greatly reduces competition for an introduced biocontrol agent. In addition, these systems are usually pathogen-free upon planting allowing the establishment of a biocontrol agent prior to pathogen introduction. Last, the temperatures, high moisture levels, and pH ranges of hydroponic systems can be ideal for the proliferation of many biocontrol agents. With all of these advantages for the use of biocontrol agents in hydroponic systems, our company, and many labs around the world, have focused their attention on developing biological control agents for these systems. I will provide a review of research focused on controlling root diseases of vegetables grown in rockwool and other hydroponic systems.
Dipak Sharma-Poudyal, Timothy C. Paulitz and Lindsey J. du Toit
Thirty-five onion genotypes were evaluated for resistance to stunting caused by Rhizoctonia solani anastomosis group 8 (AG 8) in a growth chamber set at 15 ± 1 °C. The trial was repeated. Resistance to R. solani AG 8 was defined as a lack of significant difference in plant height, root length, and/or total dry biomass between inoculated and noninoculated plants of the same genotype. Plant height was not reduced significantly by R. solani AG 8 for 14 and 7 of the 35 genotypes in Trials 1 and 2, respectively. In Trial 1, plant height reduction caused by R. solani ranged from 24% for the cv. Lasalle to 62% for the experimental line R14882, and in Trial 2 plant height reduction ranged from 22% for the experimental line PX07713218 to 53% for the cv. Montblanc. However, the extent of reduction in plant height caused by R. solani did not differ significantly among genotypes in either trial. Onion root length was not reduced by R. solani AG 8 for 26 and 18 of the 35 genotypes in Trials 1 and 2, respectively. The degree of reduction in root length ranged from 9% (R14889) to 76% (Sterling and SN232) in Trial 1, and 14% (SN325) to 74% (Sterling) in Trial 2. Onion dry biomass was not reduced by R. solani AG 8 for 19 and 7 of the 35 genotypes in Trials 1 and 2, respectively, and ranged from 18% (Elbrus) to 69% (Sterling) in Trial 1, and 29% (SN232) to 79% (Sterling) in Trial 2. The reduction in onion root length and total biomass did not differ significantly among onion genotypes in Trial 1, but differed among genotypes in Trial 2. Of the 35 genotypes evaluated, 3, 16, and 3 demonstrated partial resistance to R. solani AG 8 for plant height, root length, and total biomass, respectively, in both trials. Only four genotypes displayed partial resistance to R. solani AG 8 for at least two of the three growth parameters: plant height, root length, and biomass of PX07713218 were unaffected by the fungus in either trial; and R14885, R14888, and SN307 displayed partial resistance in both trials for two of the three parameters, and in one of the two trials for the third parameter. These four genotypes could be used in onion breeding programs to develop cultivars partially resistant to stunting caused by R. solani AG 8.
Alexis K. Nagel, Guido Schnabel, Cesar Petri and Ralph Scorza
Cox, K. Layne, D. Scorza, R. Schnabel, G. 2006 Gastrodia anti-fungal protein from the orchid Gastrodia elata confers disease resistance to root pathogens in transgenic tobacco Planta 224 1373 1383
Chitosan, a polymer of β-1,4-d-glucosamine derived from crabshell chitin, was applied to tomato plants before inoculation with the root pathogen Fusarium oxysporum f.sp. radicis-lycopersici. Whether chitosan was applied by leaf spraying, root coating, or seed coating, it reduced the number of root lesions caused by the fungus and increased the formation of physical barriers in infected root tissues. Formation of wall appositions such as papillae and occlusion of xylem vessels with coating substances were among the most typical features of host reactions. Another type of response was the accumulation of globular structures and electron-opaque masses in host cells neighboring colonized areas. Gold cytochemistry revealed that callose and lignin-like material were the main structural compounds induced in response to chitosan treatment and infection. Biochemical investigations showed that PR proteins as well as enzymes of the secondary metabolism were also significantly induced. The potential of chitosan as a biocontrol compound is discussed in relation to its ability to enhance plant protection against root pathogens.
Laurie E. Drinkwater, Deborah K. Letourneau, Fekede Workneh, Marita Cantwell, Ariena H.C. van Bruggen and Carol Shennan
Twenty commercial tomato production systems were compared in a multidisciplinary on farm study. The aim was to determine if organic (ORG) and conventional (CNV) systems differed in terms of agronomic criteria or indicators of underlying ecological characteristics. Field level measures of inputs, yields, fruit quality, arthropod abundance and management operations were made. Also, multiple samples within each field were taken to measure soil chemical and physical properties, root pathogen populations, disease incidence, and pest damage levels for multivariate analysis. Management effects on agronomic criteria (yield, fruit quality, pest damage) were small, whereas differences in soil N pools, microbial activity, pathogen populations and arthropod communities between ORG and CNV sites were sufficiently robust to be distinguished from site to site variation. Relationships between management, crop productivity and fruit quality will be discussed.
Jim Syvertsen and Yoseph Levy
Multiple stresses almost always have synergistic effects on plants. In citrus, there are direct and indirect interactions between salinity and other physical abiotic stresses like poor soil drainage, drought, irradiance, leaf temperature, and atmospheric evaporative demand. In addition, salinity interacts with biotic pests and diseases including root rot (Phytophthora spp.), nematodes, and mycorrhizae. Improving tree water relations through optimum irrigation/drainage management, maintaining nutrient balances, and decreasing evaporative demand can alleviate salt injury and decrease toxic ion accumulation. Irrigation with high salinity water not only can have direct effects on root pathogens, but salinity can also predispose citrus rootstocks to attack by root rot and nematodes. Rootstocks known to be tolerant to root rot and nematode pests can become more susceptible when irrigated with high salinity water. In addition, nematodes and mycorrhizae can affect the salt tolerance of citrus roots and may increase chloride (Cl-) uptake. Not all effects of salinity are negative, however, as moderate salinity stress can reduce physiological activity and growth, allowing citrus seedlings to survive cold stress, and can even enhance flowering after the salinity stress is relieved.
Andrew C. Ludwig, John F. Hubstenberger, Gregory C. Phillips and G. Morris Southward
Callus cultures were established from intraspecific lines of Allium cepa L., interspecific F1 progeny of A. cepa crossed to A. fistulosum L. and to A. galanthum L., advanced generations of A. fistulosum x A. cepa backcrossed to A. cepa, and lines of A. fistulosum and A. galanthum. These genotypes had been identified as susceptible, resistant, or partially resistant tester lines based on prior seedling and field nursery screenings using the pink-root pathogen Pyrenochaeta terrestris (Hansen) Gorenz, Walker and Larson. Tester line calli were challenged in vitro with culture filtrates of the fungal pathogen and were assessed by visible damage ratings expressed as the percentage of pigmentation in response to the filtrate. The degrees of callus sensitivity to the filtrate observed in vitro corresponded well with the in vivo tester line classifications. These results eliminated the possible confounding influence of using various species of Allium for in vitro screening. Our results indicated the suitability of the in vitro screening approach for the possible identification of useful segregants or somaclonal variants possessing pink-root resistance. However, in vivo pathogenicity may involve mechanisms in addition to sensitivity to the putative toxins present in the filtrate.
Brad Geary, Corey Ransom, Brad Brown, Dennis Atkinson and Saad Hafez
Onions (Allium cepa) are a high-value crop in the Treasure Valley of eastern Oregon and western Idaho. To grow quality bulbs, growers use fumigants and pesticides to control weed, pathogen, and nematode pests. Metam sodium and other commonly used pesticides are considered effective in controlling onion pests but also pose a wide range of problematic issues related to the environment, human health, and economic profitability. Biofumigant crops ‘Idagold’ mustard (Sinapis alba) and ‘Colonel’ oil seed radish (Raphanus sativus) were evaluated as substitutes for commercial fumigants in controlling soil-borne pests. During some years of this study, there was limited control of a few weed species based on visual and biomass data for metam sodium and biofumigant treatments, but the weed control was not consistent or acceptable. The severity of pink root (Phoma terrestris), a major fungal onion root pathogen, was significantly lower with metam sodium in most comparisons with other treatments. Biofumigants showed limited effects on pink root severity and are not a viable option for pink root control. Nematode damage or populations were not high enough to determine if the biofumigant and metam sodium treatments influenced their control. In some years, biofumigants reduced onion yield. The influence of these biofumigants on onion pest control was limited and their use in onion production is not warranted without significant nutrient recycling or soil quality benefits.