- Author: Mark Bolda
The element sulfur (S) has a large role in the management of plant disease. Growers are familiar with the biocidal formulations of sulfur, being elemental sulfur, sulfides, thiosulfate and fumigants like dimethyldisulfide (DMDS) and of course then we have sulfur dioxide which has been used as a postharvest preservative for dried fruits and vegetables.
However, beyond these outright biocidal effects we get from use of sulfur as a fungicide, there is also significant literature concerning the indirect effects of sulfur nutrition on reinforcing a plant's ability to inhibit and resist disease. Known as “Sulfur Induced Resistance” this is how one should frame the role of sulfur incorporated as a nutrient in plant response to disease.
There could be something to this. Work has been done showing that higher rates of S fertility affected infection rates and severity of fungal disease in oilseed rape and stem canker of potato. While informative, it is significant that the results of the former were achieved by sulfur additions to a field that was deficient.
How would sulfur induce resistance or inhibit disease in a plant? Sulfur goes to many places, including the amino acids acids cysteine and methionine, which are in 99% of proteins found in a plant. Findings cited by the chapter report that sulfur deficiency in the plant result in lower protein bound cysteine and free cysteine, which as the precursor to all relevant sulfur containing metabolites must have something to do with the ability to resist or inhibit disease.
Sulfur also goes to non-protein reservoirs in the plant, one of the main ones being glutathione. Glutathione, known as a phytoalexin because it is not formed prior to disease incursion, nevertheless accumulates rapidly after pathogen attack. It is involved in detoxifying signals necessary for fungal growth and could also be serving as a messenger to carry information to yet unaffected plant cells.
Phytoanticipins, in contrast to phytoalexins, are molecules in the plant which are preformed antibiotics- i.e. the plant produces them whether or not there is disease. Glucosinolates (of which our well known isothiocyanates are a cleaved product), on which sulfur plant nutrition has a strong influence, are one of these antibiotic phytoanticipins. Interestingly, low concentrations of glucosinolates don't necessarily equate with higher disease susceptibility, making them more of a qualitative defense for the plant.
Bottom line: The role of sulfur in disease resistance and inhibition in plants is a very important one, but it's a pretty sure thing that these systems function perfectly well in sulfur sufficient soils, which describe pretty well every one in the Pajaro and Salinas Valleys. As such, while the value of sulfur as a foliar fungicide is indisputable for certain diseases, I am not seeing the value of pursuing sulfur work experimentally as a soil disease mitigant.
The above is a summary of some of the aspects of Chapter 8: Sulfur in Plant Disease from “Mineral Nutrition and Plant Disease” edited by Lawrence Datnoff, Wade Elmer and Don Huber.