J.C. Sutton

Etiology and epidemiology of pythium root rot in hydroponic crops : Current knowledge and perspectives

The etiology and epidemiology of Pythium root rot in hydroponically-grown crops are reviewed with emphasis on knowledge and concepts considered important for managing the disease in commercial greenhouses. Pythium root rot continually threatens the productivity of numerous kinds of crops in hydroponic systems around the world including cucumber, tomato, sweet pepper, spinach, lettuce, nasturtium, arugula, rose, and chrysanthemum. Principal causal agents include Pythium aphanidermatum, Pythium dissotocum, members of Pythium group F, and Pythium ultimum var. ultimum. Perspectives are given of sources of initial inoculum of Pythium spp. in hydroponic systems, of infection and colonization of roots by the pathogens, symptom development and inoculum production in host roots, and inoculum dispersal in nutrient solutions. Recent findings that a specific elicitor produced by P. aphanidermatum may trigger necrosis (browning) of the roots and the transition from biotrophic to necrotrophic infection are considered. Effects on root rot epidemics of host factors (disease susceptibility, phenological growth stage, root exudates and phenolic substances), the root environment (rooting media, concentrations of dissolved oxygen and phenolic substances in the nutrient solution, microbial communities and temperature) and human interferences (cropping practices and control measures) are reviewed. Recent findings on predisposition of roots to Pythium attack by environmental stress factors are highlighted. The commonly minor impact on epidemics of measures to disinfest nutrient solution as it recirculates outside the crop is contrasted with the impact of treatments that suppress Pythium in the roots and root zone of the crop. New discoveries that infection of roots by P. aphanidermatum markedly slows the increase in leaf area and whole-plant carbon gain without significant effect on the efficiency of photosynthesis per unit area of leaf are noted. The platform of knowledge and understanding of the etiology and epidemiology of root rot, and its effects on the physiology of the whole plant, are discussed in relation to new research directions and development of better practices to manage the disease in hydroponic crops. Focus is on methods and technologies for tracking Pythium and root rot, and on developing, integrating, and optimizing treatments to suppress the pathogen in the root zone and progress of root rot.

Interactions among Pythium aphanidermatum, roots, root mucilage, and microbial agents in hydroponic cucumbers

Epidemics and biological control of Pythium root rot were investigated in greenhouse cucumbers grown in small-scale hydroponic systems with troughs and recirculating nutrient solution. The systems were inoculated with Pythium aphanidermatum only or with the pathogen plus an isolate of Pseudomonas fluorescens or Pseudomonas chlororaphis. In systems with P. aphanidermatum only, growth rate of roots, amount of root mucilage in the nutrient solution, and root discoloration (browning) oscillated in patterns that were generally similar and synchronous. These variables abruptly increased for 6-14 days, starting at about 21 days after transplanting, subsequently declined to low levels at 41-56 days, and then increased again starting at 53-61 days. Logarithm to the base 10 of propagule density of P aphanidermatum usually was several orders higher in the mucilage (2.09-4.90/g) than in the nutrient solution (0.25-2.67/mL). The pathogen was also more frequent in roots with associated mucilage (14-60 colonies/60 cm of root) compared to those lacking mucilage (2-26 colonies/60 cm of root). These findings, combined with positive correlation between mucilage and discolored roots, and microscopic observations of Pythium hyphae in mucilage and discolored roots, indicated that the mucilage supported prolific growth of P. aphanidermatum and served as a food base from which the pathogen invaded roots. The bacterial agents each reduced root mucilage until about 68 days after transplanting, and root discoloration chiefly at 47-61 days. Mucilage breakdown by the agents was an apparent mode of biological control of P. aphanidermatum. Pseudomonas fluorescens reduced mucilage and root discoloration more effectively than P. chlororaphis, but did not significantly promote marketable yield (fresh mass basis). Pseudomonas chlororaphis increased cumulative marketable fruit yield at 47-78 days, but not at 78-127 days. Both agents increased numbers of marketable fruit. The agents did not affect incidence of the pathogen in young roots, which are normally infected by zoospores. Densities of incidental microbes in nutrient solution were high after 69-74 days. It is hypothesized that shifts in assimilate partitioning to roots successively affected root growth, mucilage production, saprotrophic growth of P. aphanidermatum, and root invasion by hyphae of the pathogen. The microbial agents appear to have value for mucilage management and root rot control.

Biological control ofBotrytis cinerea in residues and flowers of Rose (Rosa hybrida)

Microbial isolates from living petals, petal residues and leaf residues of rose, and from laboratory collections, were evaluated for control ofBotrytis cinerea in rose. In leaf residues artificially infested withB. cinerea, isolates of the filamentous fungiGliocladium roseum, FR136 (unidentified) andTrichoderma inhamatum reduced sporulation of the pathogen by >90%, other filamentous fungi were 25–90% effective, and those of yeasts and bacteria were <50% effective. In artificially inoculated petal residues, no microbe reduced sporulation ofB. cinerea by >75%, but isolates ofCladosporium oxysporum and four yeasts were 51–75% effective, and three filamentous fungi, eight yeasts andBacillus subtilis isolates were 26–50% effective. Isolates ofT. inhamatum, C. oxysporum andG. roseum performed best againstB. cinerea among isolates evaluated in leaf residues naturally infested with the pathogen and indigenous microorganisms. Totals of ten isolates of filamentous fungi (includingC. oxysporum andC. cladosporioides), two of yeasts and five ofBacillus subtilis completely prevented lesion production byB. cinerea in detached petals, and a further six isolates of filamentous fungi (includingG. roseum) and six yeasts were 90–99% effective. Isolates ofC. oxysporum, C. cladosporioides andB. subtilis, the most effective microorganisms againstB. cinerea in flower buds, reduced number of lesions in the range of 42–65% compared with 59–89% for à standard fungicide (vinclozolin). It is suggested that application of leading antagonists Jo living rose leaves and flowers should optimize control of inoculum production byB. cinerea when the tissues die. Optimal biocontrol of lesion production in flower buds requires a better understanding of the microenvironment of petals.

Ability of Clonostachys rosea to Establish and Suppress Sporulation Potential of Botrytis cinerea in Deleafed Stems of Hydroponic Greenhouse Tomatoes

The ability of Clonostachys rosea to establish and persist in deleafed tomato stems and to suppress sporulation potential of Botrytis cinerea was investigated in plots of hydroponic tomatoes in commercial greenhouses. Leaves near lower fruit clusters were removed according to standard practice and deleafed portions of the stems were treated with C. rosea , iprodione or water. Inoculum of B. cinerea was from natural infections. Stem lesions were not produced by the pathogen during the trials. Development of C. rosea and B. cinerea in stems was estimated indirectly by quantifying sporulation on excised stem tissues that were incubated on an agar medium containing paraquat. Incidence and area of sporulation of C. rosea on tissue pieces were high (76-99%) and moderately high (33-79%), respectively, when stems were treated with the agent at 0, 6, 24 or 48 h after deleafing and sampled 11 to 75 days later. In various instances, the agent also sporulated on tissues from water controls and iprodione treatments, apparently after interplot transmission. In most instances, incidence and area of sporulation of B. cinerea on tissue pieces were high (83-100%) and moderate to high (35-76%), respectively, in the water controls, but moderate (31-44%) and moderate to low (5-34%), respectively, for stems treated with C. rosea at 0 to 48 h after deleafing and sampled after 11-75 days. Without exception, C. rosea suppressed B. cinerea as or more effectively than iprodione. Correlations between inoculum density of C. rosea (0-10 6 conidia mL -1 ) and sporulation potential of B. cinerea in deleafed stems were strongly negative in each of three tests ( r = -0.95 to -0.99). Conidial suspensions and a talc formulation of C. rosea were of similar effectiveness against B. cinerea . We conclude that C. rosea persisted and suppressed sporulation potential of B. cinerea in deleafed tomato stems for at least 11 weeks after application.

Relationships of Pythium isolates and sweet pepper plants in single-plant hydroponic units

An isolate of Pythium aphanidermatum and five isolates of Pythium dissotocum were examined for ability to colonize and produce symptoms in root systems of sweet pepper grown in aerated nutrient solution in single-plant hydroponic units in a research greenhouse. Repetitions of the study were conducted in April 2000, February 2001, and May 2001. Roots were inoculated by immersion for 30 min in a suspension of 5 × 103 zoospores/mL nutrient solution. All isolates rapidly colonized the root systems and increased density of cortical cells with cytoplasmic granulation in all repetitions. Symptoms produced by a given isolate differed but generally overlapped with those caused by other isolates and variously included zones of root-tip browning of different sizes and hues (all isolates), expansive browning (P. aphanidermatum) or yellowing (P. dissotocum), architectural changes (chiefly stunting, stubbiness, and proliferation of roots with all isolates), root swelling (P. dissotocum), and callus cell proliferation (P. dissotocum). Patterns of increase in incidence of brown root tips and of expansive root discoloration differed among isolates and among experimental repetitions. Circumstantial evidence suggested that higher temperature and light conditions of May 2001 promoted disease caused by P. aphanidermatum and P. dissotocum isolates compared with February 2001 and, in some instances, April 2000. Observations indicated that moderate differences in temperature, light, or other environmental variables among the repetitions markedly influenced the expression of discoloration and of other symptoms in colonized roots. All isolates reduced various growth parameters of the roots and shoots in one or more repetitions; however, relationships of colonization and symptoms to growth were generally unclear. Collectively, the data have applications for evaluating control methods in single-plant hydroponic units and contribute to an understanding of the epidemiology of root rot caused by Pythium spp. in greenhouse pepper.

Effects of Pseudomonas chlororaphis on Pythium aphanidermatum and Root Rot in Peppers Grown in Small-scale Hydroponic Troughs

The ability of Pseudomonas (Ps.) chlororaphis isolate Tx-1 to suppress Pythium aphanidermatum and control root rot was investigated in sweet peppers grown in small-scale hydroponic trough units with recirculating nutrient solution. The agent was introduced to the nutrient solution 3 days after the peppers were inoculated with P. aphanidermatum, or 4 days before and 3 days after inoculation, or 4 days before and 3 and 10 days after. Controls either received no agent or pathogen, or the agent only (applied once, 4 days before the time of pathogen inoculations), or the pathogen only. The experiment was conducted in a greenhouse with repetitions beginning in January, March, and May. Severity of root browning associated with P. aphanidermatum, in the absence of Ps. chlororaphis, increased in the first, second and third repetitions, respectively, to 74% at 21 days, 28% at 21 days, and 68% at 11 days. In treatments with one, two, or three applications of Ps. chlororaphis, respectively, areas under root browning progress curves were reduced by 18-48%, 50-73%, and 62-79% compared to the pathogen controls, and recovery incidence of the pathogen from roots at 21 days after inoculation was reduced by 14-47%, 62-82%, and 60-89%. Roots not inoculated with P. aphanidermatum were whitish and not colonized by the pathogen. Density of Ps. chlororaphis in treated plants was usually 2×103-1.2×105 cfu g−1 fresh roots. The Ps. chlororaphis treatments almost invariably prevented reductions in plant height, leaf area, fresh mass and dry mass of the shoots and of the roots, and root volume associated with P. aphanidermatum and root browning. It is concluded that Ps. chlororaphis Tx-1 strongly antagonized P. aphanidermatum in the pepper roots and that the agent has considerable potential for controlling root rot and maintaining productivity in commercial hydroponic peppers.

Development ofClonostachys rosea and interactions withBotrytis cinerea in rose leaves and residues

The effect of microclimate variables on development ofClonostachys rosea and biocontrol ofBotrytis cinerea was investigated on rose leaves and crop residues. C.rosea established and sporulated abundantly on inoculated leaflets incubated for 7–35 days at 10°, 20° and 30°C and then placed on paraquat—chloramphenical agar (PCA) for 15 days at 20°C. On leaflets kept at 10°C, the sporulation after incubation on PCA increased from 60% to 93% on samples taken 7 to 21 days after inoculation, but decreased to 45% on material sampled after 35 days. A similar pattern was observed on leaves incubated at either 20° or 30°C. The sporulation ofC. rosea on leaf disks on PCA was not affected when the onset of high humidity occurred 0, 4, 8, 12 or 16 h after inoculation. However, sporulation was reduced to 54–58% on leaflets kept for 20–24 h under dry conditions after inoculation and before being placed on PCA. The fungus sporulated on 68–74% of the surface of leaf disks kept for up to 24 h at high humidity after inoculation, but decreased to 40–51% if the high humidity period before transferral to PCA was prolonged to 36–48 h. The growth ofC. rosea on leaflets was reduced at low inoculum concentrations (103 and 104 conidia/ml) because of competition with indigenous microorganisms, but at higher concentrations (105 and 106 conidia/ml) the indigenous fungi were inhibited. Regardless of the time of application ofC. rosea in relation toB. cinerea, the pathogen’s sporulation was reduced by more than 99%. The antagonist was able to parasitize hyphae and conidiophores ofB. cinerea in the leaf residues. AsC. rosea exhibited flexibility in association with rose leaves under a wide range of microclimatic conditions, and in reducingB. cinerea sporulation on rose leaves and residues, it can be expected to suppress the pathogen effectively in rose production systems.

Inoculum of Pyrenophora tritici-repentis in relation to epidemics of tan spot of winter wheat in Ontario

Inoculum, host development, weather factors, and disease were monitored in 1986-87 and 1987-88 as components of tan spot epidemics in winter wheat, caused by Pyrenophora tritici-repentis. Development of pseudothecia on wheat stubble residues exhibited a seasonal pattern in which the ascocarps enlarged during August-October, asci formed during December-March, and ascospores differentiated beginning in late February or March. Mature pseudothecia capable of releasing ascospores were found from early April to mid June, coinciding with wheat growth stages from tillering to anthesis or medium milk development. Significant linear relationships were found between incidence of pseudothecia that had matured and degree-day accumulation (base temperature 0°C). Conidia of the pathogen dispersed from diseased wheat leaves were trapped chiefly in June and July. From observations of inoculum and incidence of lesions, the epidemics were characterized by a prolonged simple-interest phase initiated by ascospores followed by...

Relationships of temperature, moisture, and inoculum density to the infection cycle of Peronospora destructor

(1984). Relationships of temperature, moisture, and inoculum density to the infection cycle of Peronospora destructor. Canadian Journal of Plant Pathology: Vol. 6, No. 2, pp. 127-134.

DENSITY DYNAMICS OF GLIOCLADIUM ROSEUM IN RELATION TO BIOLOGICAL CONTROL OF BOTRYTIS CINEREA IN RED RASPBERRY

Abstract Density patterns of colony-forming units (CFU) of the biocontrol agent Gliocladium roseum were investigated in raspberry cv. Boyne and cv. Redwing after spore suspensions (107 conidia/mL) were applied 2 h before dusk in field plots. Estimated density of G. roseum on leaves of both cultivars was 2 x 103 to 5 x 103 CFU/cm2 at 1 h after inoculation. During the next 2 days density of the agent progressively decreased, but at 3–10 days after inoculation it fluctuated around 10–102 CFU/cm2 leaf in ‘Boyne’ and after or 3–12 days, it fluctuated near 103 CFU/cm2 leaf in ‘Redwing’. Density of G. roseum on flowers was 1 x 104 to 5 x 104 CFU/flower at 1 h after inoculation and declined rapidly in ‘Boyne’ and slowly in ‘Redwing’. At 3–10 and 3–12 days, respectively, the density fluctuated around 70 CFU/flower in ‘Boyne’ and 800–2300 CFU/flower in ‘Redwing’. Germination of G. roseum, observed on stamens of ‘Boyne’, was sparse (<4–10%) in the field. However, germination potential, estimated in stamens of flower...