Physical and cultural methods for the management of soil-borne pathogens

There is an increasing interest in using physical methods and cultural practices (CP) in disease control as alternatives to pesticides for the management of soil-borne pathogens. These can be used alone or as components of pest management programs. In this regard, there are three categories of CP: (a) CP for regular purposes which can also be used for disease control, e.g. irrigation; (b) CP which are used solely or mainly for pest control, e.g. sanitation; (c) CP which can be used for both agricultural purposes and pest control, e.g. crop rotation. The basic principles of CP for pest control are (a) any potential control method may be considered, providing that it is environmentally, technologically and economically feasible; (b) pesticide usage is minimized by combining with other non-chemical or chemical methods; (c) diseases that are difficult to control or that involve problematic pesticides, e.g. methyl bromide, should be prioritized; (d) economic aspects are taken into consideration. Physical methods include heating the soil or propagation material, irradiation, etc. CP for pest control can be used before, at or after planting. They include crop rotation, fallow, flooding, deep ploughing, flaming, soil solarization — which involves a combination of physical and biological processes, adjusting planting date, irrigation, fertilization, compost, weed control, herbicide application, sanitation, tillage and others.     

Effect of moisture on thermal inactivation of soilborne pathogens under structural solarization

ABSTRACT Structural solarization of greenhouses for sanitation by closing them involves dry heating to 60 degrees C and higher with a consequent low relative humidity (RH) ( approximately 15%), thus requiring an extended period for thermal inactivation of pathogens. In an attempt to enhance pathogen control by increasing moisture during the hot hours of the day, various regimes of inoculum moistening were studied. However, wetting inoculum of Fusarium oxysporum f. sp. melonis and F. oxysporum f. sp. radicis-lycopersici resulted in less effective pathogen control compared with that of dry heating. Fifty percent effective dose (ED(50)) values of thermal inactivation of wetted and dry inoculum for the former pathogen were 18 and 7 days, respectively, and for the latter, a respective 9 and 4 days. This was because wetting resulted in inoculum cooling due to evaporation, which eventually led to its drying. A model describing the drying of wet inoculum in a wetted greenhouse, based on the fact that there was an approximately 10 degrees C difference between greenhouse and ambient temperatures, was proposed. A double-tent system reduced this difference to 1 to 2 degrees C, reduced moisture loss, and led to improved inoculum inactivation of F. oxysporum f. sp. radicis-lycopersici. Thus, the ED(50) value of thermal inactivation was reduced from 15 days to 1 day, because this system provided both high temperature ( approximately 60 degrees C) and high RH ( approximately 100%), resulting in effective wet heating.     

Plant disease management in organic farming systems

Organic farming (OF) has significantly increased in importance in recent decades. Disease management in OF is largely based on the maintenance of biological diversity and soil health by balanced crop rotations, including nitrogen‐fixing and cover crops, intercrops, additions of manure and compost and reductions in soil tillage. Most soil‐borne diseases are naturally suppressed, while foliar diseases can sometimes be problematic. Only when a severe disease outbreak is expected are pesticides used that are approved for OF. A detailed overview is given of cultural and biological control measures. Attention is also given to regulated pesticides. We conclude that a systems approach to disease management is required, and that interdisciplinary research is needed to solve lingering disease problems, especially for OF in the tropics. Some of the organic regulations are in need of revision in close collaboration with various stakeholders. © 2015 Society of Chemical Industry     

Combined soil treatments and sequence of application in improving the control of soilborne pathogens

ABSTRACT The effects of reduced doses of methyl bromide (MB) or metham sodium, heating, short solarization, and soil microbial activity, alone or in combination, on survival of soilborne fungal pathogens were tested in a controlled-environment system and field plots. Sublethal doses of heating or MB delayed germination of Sclerotium rolfsii sclerotia. Combining MB and heating treatments was more effective than either treatment alone in controlling S. rolfsii and Fusarium oxysporum f. sp. basilici. The application heating followed by fumigation with MB, was significantly more effective in delaying and reducing germination of S. rolfsii sclerotia and in controlling F. oxysporum f. sp. basilici than the opposite sequence. Further, incubation in soil and exposure to microbial activity of previously heated or MB-treated sclerotia increased the mortality rate, indicating a weakening effect. Similarly, incubation of chlamydospores of F. oxysporum f. sp. melonis and F. oxysporum f. sp. radicis-lycopersici in soil in the field after fumigation further reduced their survival, confirming the laboratory results. In field tests, combining MB or metham sodium at reduced doses with short solarization was more effective in controlling fungal pathogens than either treatment alone. Treatment sequence significantly affected pathogen control in the field, similar to its effect under controlled conditions. This study demonstrates a frequent synergistic effect of combining soil treatments and its potential for improving pathogen control and reducing pesticide dose, especially when an appropriate sequence was followed.     

Chemical control of soil-borne pathogens in tuff medium for strawberry cultivation

The phytopathogenic fungi Fusarium oxysporum f. sp. lycopersici and Rhizoctonia solani were completely eradicated after exposure for 30 min to a 0.4-0.5 % aqueous solution by weight of formaldehyde. With metham-sodium, higher concentrations or longer exposures were needed to obtain similar results. When tested in tuff (a granular plant growth medium of volcanic rock origin), the toxicity of formaldehyde was lower and that of metham-sodium was higher, compared with the corresponding toxicities in aqueous solutions. Both fungicides eradicated the two fungi, as well as Pythium myriotilum, when tested under practical conditions in vertical aluminium tubes filled with tuff. No detrimental effect could be observed on strawberry plants subsequently grown in these columns.     

Survival of plant pathogens under structural solarization

Structural solarization of greenhouses is a nonchemical sanitation procedure. The method involves dry heating, since maximal temperatures may exceed 60°C and consequent relative humidity (r.h.) is low (ca 15%), under fluctuating temperature and r.h. regimes. Thirty-five structural solarization experiments were performed over 7 years, testing one bacterial and five fungal plant pathogens. Various aspects of pathogen thermal inactivation under this method were studied. Thermal inactivation of the various pathogens differed according to the organism and inoculum form. Sensitivity to heat was highest withClavibacter michiganensis subsp.michiganensis and lowest withFusarium oxysporum f.sp.radicislycopersici inoculum in dry infected tomato stems, with ED₈₀ values of 7 and 47 days, respectively; intermediate values were obtained forPythium sp.,F. oxysporum f.sp.melonis, F. oxysporum f.sp.basilici andSclerotium rolfsii. The maximal ambient temperatures were in the range of 28.2° to 33.1°C. Structural solarization for sanitation can be a useful component of integrated pest management in greenhouses. http://www.phytoparasitica.org posting Sept. 28, 2004.     

Toxicity of chloronitrobenzenes toFusarium oxysporum and rhizoctonia solani as related to their structure

The structure-activity relationship of several chlorinated nitrobenzenes was studied using two soilborne fungi,Fusarium oxysporum f. sp.melonis Snyder and Hansen andRhizoctonia solarii Kühn. Fungitoxicity increased with the increase in number of chlorine substituents and was also affected by the position of the halogens on the phenyl ring. A linear relationship was obtained when the fungitoxicity values (EC₅₀) of the compounds were plotted against their lipophilicity values calculated from octanol-water partition coefficient π.R.solarii was much more sensitive thanF. oxysporum to chloronitrobenzenes, particularly with respect to the pentachloro derivative.     

Controlled laboratory system to study soil solarization and organic amendment effects on plant pathogens

ABSTRACT A controlled laboratory system for simulating soil solarization, with and without organic amendment, was developed and validated using physical, chemical, and biological parameters. The system consists of soil containers that are exposed to controlled and constant aeration, and to temperature fluctuations that resemble those occurring during solarization at various depths. This system enables a separate analysis of volatiles and other components. We recorded a sharp decrease in oxygen concentration in the soil atmosphere followed by a gradual increase to the original concentration during solarization in the field and heating in the simulation system of soil amended with wild rocket (Diplotaxis tenuifolia) or thyme (Thymus vulgaris). The combined treatment of organic amendment and solarization (or heating in the controlled system) was highly effective at controlling populations of Fusarium oxysporum f. sp. radicis-lycopersici. Changes in soil pH, enzymatic activities, and microbial populations followed, in most cases, trends which were similar under both solarization and the heating system, when exposed to controlled aerobic conditions. The reliability and validity of the system in simulating physical, chemical, and biological processes taking place during solarization is demonstrated.