Biology, ecology and control of soil-borne plant pathogens, Soil solarization.
Highlights on Recent Studies and Progress in Soil Solarization
Summarized by J. Katan, A. Grinstein and A. Gamliel
Soil solarization research has been carried out in more than 50 countries. The research and development in this field are expanding in many directions. We do not attempt to cover all of them, but rather to report briefly some examples from five areas. Additional reports will be given in the future. You are invited to send us information, comments and suggestions, and these will be referred to in future reports.
We would like to emphasize that the following is a very brief report (therefore, names of researchers have not been mentioned) describing some of the developments in solarization research. There are many other developments, not less important, which will be referred to in future reports, if we feel that there is an interest. Your help and suggestions in this regard will be much appreciated. We should be happy that solarization research has expanded so much that it is not possible to cover all of the recent developments in one report.
More information on soil solarization is given in the next pages.
1. Soil solarization worldwide
In addition to the regular studies on the effectiveness of soil solarization in controlling a variety of pests of major crops in regions which are especially suitable for solarization, this technique is being tested continuously in new regions and countries and with additional crops and pests. For example, there have been studies on the effectiveness of solarization in controlling phytopathogenic bacteria, Urocystis in onions, foliar diseases, soil arthropods and others. It has recently been found in India that the Karnal bunt pathogen of wheat (a threatening disease) is very sensitive to solarization. We do not expect solarization to become a regular tool for controlling this disease in wheat but it can be considered in emergency cases for eradicating the pathogen in a new region. Other innovative studies deal with disinfecting water by solar heating, which has a useful potential in remote areas, and the use of solar chambers for tree treatment. Success has also been reported in the control of soilborne pathogens in existing orchards, although the main efforts are concentrated on adopting solarization for the control of major soilborne pathogens and weeds in annual crops. A remarkable rapid development in solarization research and development has been observed in Latin America and India. In the latter, efforts have been made to introduce solarization to nurseries and to combine it with organic amendments (see below). There are additional reports on the use of solarization in nurseries. In Florida, where extensive studies on solarization are being conducted, the efforts have been successfully directed to adapting solarization to the local crop management system, especially with tomatoes. The use of solarization for controlling parasitic plants is an another topic of solarization research. A fundamental issue in solarization research in every region is the determination of the months which are optimal for solarization from the climatic point of view, without interfering with crop management. In order to address this issue, data on the climate and on thermal sensitivity of pathogens are required, as well as physical and biological simulation models related to heating of soil and pathogens.
With the upcoming reduction in the use of methyl bromide and its consequent phase out, soil solarization research is expected to further intensify and its use to be expanded. After all, at this stage, except for artificial soil heating, soil solarization (alone or combined with other means) is the only nonchemical soil disinfestation method which has been tested on a large scale under farming conditions. It can potentially to replace many (but not all) uses of methyl bromide. However, the full realization of its potential requires a systematic and intensive research.
2. Improving soil solarization
A major limitation of this method is its dependency on climate. A better reliability in its use and a higher effectiveness in pest control will possibly enable its usage for additional months during the year and in additional climatically marginal regions. For example, the use of solarization in a closed glasshouse (a well-known approach for improving solarization) resulted in a very effective control of Plasmodiophora even in a northern area, the Czech Republic, which is not a typical site for solarization. The main effort for improving solarization has focused in recent years on combining it with reduced dosage of pesticides and with biocontrol agents, but especially with organic amendments which produce volatiles accumulating under the plastic mulch. Crucifers are very effective, but other organic amendments can also be used. This approach, also referred to as "biofumiagation", is followed by many groups of researchers including groups in Europe and Canada. The mode of action, with emphasis on volatile production, is being studied. We believe that the efforts should focus on available and cheap organic materials, such as poultry manure and local residues, or on organic wastes which are an ecological burden, e.g. surplus from the food industry. Improving solarization technologies for better heating is an another field of research which can contribute to the improvement of solarization.
3. Mechanisms and mode of action
The biological processes involved in solarization continue to draw interest, especially among the younger generation of researchers and graduate students, who are fortunately much involved in solarzation research. These processes include the weakening effect on pathogen propagules induced by sublethal heating (which can lead to induced biological control and has many implications on combining solarization with other methods) and the shifts in populations of native biocontrol agents such as Talaromyces, Aspergillus, Bacillus and Pseudomonas in soil. Studies on effect of solarization on VAM have been conducted. A model describing solar heating in rainy areas has been developed for the first time has been and will hopefully contribute to a better use of solarization. Studies on chemical changes in the soil and on physiological (including hormonal) changes in plants growing in solarized soils, have been reported.
4. Implementation studies
These include mulching individual elevated beds (Florida), improved plastic films the use of sprayable mulches (California), Israel) and others. Economic analyses has been reported.
5. Education, Extension and International Meetings
Leaflets, brochures, bulletins and special publications in various languages, describing solarization and its use, being published continuously. Recently, a special bulletin on soil solarization with emphasis on implementation, by Elmore et al from the University of California, was published.
Our colleagues from various countries report on special farmers' days conducted to demonstrate solarization. Two video films on solarization (in one cassette) have been produced by the Extension Service of the Ministry of Agriculture in Israel. They are available in several languages (Hebrew, English, Arabic, Spanish, Portuguese, French, Italian, Russian).
Solarization studies are presented in many national and international meetings dealing with crop protection, horticulture and related subjects. Two recent conferences are the International Congress for Plastics in Agriculture which was held in Israel in 1997, and the conference on soil solarization and integrated management of soilborne pests that was held in Aleppo, Syria in 1997. Abstracts of the latter meeting are available on the Internet.