Roni Cohen, Newe Ya’ar Research Center
No tillage or minimal tillage is important for maintaining undisturbed physical structure of the soil, soil moisture, saving energy and may help elevate yield. Nevertheless, soil management can effect positively or negatively the extent of plant diseases. Plants are exposed to various pathogens, thus the above statement is too general to be accurate. Likewise, the no-tillage effect on plant disease is too broad a topic to be covered in a short article. The purpose of this short article is to raise interest on this topic by focusing on the charcoal rot disease caused by the soil-borne fungus Macrophomina phaseolina as a case study.
What are the factors affecting soil-borne disease development?
Soil tillage is only one factor affecting the incidence and severity of soil-borne disease (Fig. 1). Each interaction between plant, pathogen and environment requires specific attention. When dealing with the effect of soil tillage on pathogen survival among growing seasons, for example, it is necessary to consider different approaches to different pathogens. One pathogen may attack only the foliage, Alternaria solani for example, whilst another may attack both foliage and roots, such as the Pytophthora species attacking potato, tomato or pepper, whilst another pathogen may attack only the roots.
Likewise, the approach toward “root diseases” is not similar to that of “foliar diseases”. The variation in the fungal survival organs, the way they survive in the soil, longevity of survival time and the pathogenesis mode are different, making the relationship between soil tillage and the risk for disease initiation a complicated question.
Fig. 1. Factors affecting incidence and severity of soil-borne diseases
The charcoal rot disease caused by the fungus Macrophomina phaseolina as a case study
Macrophomina phaseolina is a soil-borne fungus known as a causal agent of charcoal rot disease of about 500 plant species. The pathogen penetrates the roots and causes wilt by macerating the plant tissues and/or poisoning it with toxins. The pathogen produces on the dead-plant tissues huge amounts of micro-sclerotia capable of persisting in the soil for years and serving as an inoculum source for plants that will grow in the field in the next cropping season (Fig. 2).
Fig. 2. Root segment of a sesame plant that wilted following Macrophomina phaseolina attack.
The black dots are micro-sclerotia capable of persisting on the root or in the soil for years and may risk the next crop. Photograph by Shira Gal
The effect of soil tillage on inoculum load in the soil and on disease incidence in soybeans and watermelon
In general, organisms produce huge amounts of multiple units such as plant pollen, sperm, spores, etc., to ensure their survival. Macrophomina phaseolina, like other fungi, produces huge numbers of micro-sclerotia on dead tissues, as the inoculum source for infesting the next crop (Fig. 2). This extra amount of inoculum in the soil raises the question as to what extent if any soil tillage or burying the dead plant tissues affects plant infection. Another question is how this inoculum level in the soil affects disease incidence and severity.
The interaction between Macrophomina phaseolina and soybeans was investigated in the U.S.A. Several studies revealed that different soil tillage methods (plowing, disking) did not affect infection sites on the roots. The number of sclerotia in the soil did not correlate with disease incidence or severity. The meaning is that small amounts of inoculum and suitable environmental conditions are enough to generate disease. From an epidemiological point of view, it is clear that some reduction in the inoculum load will not prevent disease initiation and development. It should be emphasized that those conclusions are true for this specific disease and can completely differ for others.
In an experiment conducted at the ‘Eden Experiment Station in the Bet She’an Valley, eastern Israel, watermelon seedlings were planted in soil that was either plowed or did not get any treatment (no-tillage). The hypothesis was that deep plowing will bury plant debris with the pathogen survival bodies and will help to reduce the inoculum load and disease incidence and severity. Indeed, soil treatment affected root colonization. Higher colonization was evident in the no-tillage treatment, indicating higher inoculum load compared to the deep plowing treatment. Nevertheless, no differences in disease incidence and severity were found between the two treatments. The meaning is that the available inoculum load in the soil was enough to incite disease without the need for a high inoculum load.
The relationship between soil tillage and the incidence and severity of soil-borne diseases is not simple and many factors may affect it (Fig. 1). In the specific disease that served as an example, that caused by Macrophomina phasolina in soybean and watermelon, no differences were observed between the tilling and no-tilling treatments. It should be emphasized that each plant-pathogen interaction has its own specific factors and soil tillage is only one of them. It would be best to combine no or minimal tillage (Fig. 3) with other methods and approaches of crop management, such as crop rotation, biological and chemical controls, and genetically resistant cultivars to achieve best results.
Fig. 3. No-tillage double cropping. Cotton sown directly in a wheat field that was harvested
shortly earlier. Photograph by Ethan Sela