Can Plants Get “Immunized” Against Diseases?
Exploiting plants’ natural ability to ward off pests is an alternative option to disease control that can reduce pesticide use. For decades, biologists have known that plants can develop “immunity” to certain pathogens after being “vaccinated” with microorganisms or specialized proteins. This type of resistance is technically referred to as systemic acquired resistance, and it is only recently that development of products for this type of plant protection has increased.
HOW IT WORKS
Systemic acquired resistance (SAR) is a defense response that can occur by inoculating a plant with a SAR elicitor, which could be a weaker or non-virulent strain of a specific pathogen, or a synthetic chemical compound. The resistance is regulated by salicylic acid, which drives a set of complex pathways that “jump into action” to prevent infection. Salicylic acid induces the accumulation of proteins, called pathogenesis-related proteins (PR proteins) throughout the plant, that in turn protect it from disease. The PR proteins create morphological and biochemical changes in SAR-protected plants, including a faster lignification response, an increase in glucose and fructose, and an accumulation of antimicrobial and fatty acid derivatives.
BIOLOGICAL SAR Elicitors
There are several organisms that have been examined as eliciting resistance in agricultural crops, often leading to defense against multiple organisms. For example, it was shown that radishes inoculated with a biocontrol strain of Pseudomonas fluroescens were protected from a fungal root rot and bacterial and fungal leaf spots, and cucumber inoculated with Colletotrichum lagenarium were protected against a dozen diseases caused by fungal, bacterial, and viral pathogens.
CHEMICAL SAR Elicitors
SAR can also be triggered by exposing the plant to salicylic acid. Applying salicylic acid to foliage, however, causes severe phytotoxicity, and several other chemicals have been developed that are just as effective and safer to use. The most commonly tested chemical is benzo (1,2,3) thiadiazole-7-carbothioic acid S-methyl ester (BTH). BTH (Actigard) is registered on a variety of agricultural crops. Other examples include harpin (Messenger), which is a protein derived from Erwinia amylovora, and a variety of phosphonate products that are registered on agricultural crops, turf, and ornamentals.
Phosphonates (Aliette, Agri-fos, Vital, Fosphite, etc.) are primarily used for prevention of Phytophthora diseases. Recent reports, however, show that these products have some efficacy against downy mildew, apple scab, phomopsis canker on grapes, rust on bean, fusarium wilt, dollar spot on turf, and powdery mildew.
CONCLUSIONThe use of SAR elicitors in agricultural crops and landscapes has the potential to become a sustainable approach for plant protection. Biological or chemical elicitors can effectively reduce diseases caused by a broad spectrum of pathogens. SAR can reduce the use of pesticides, does not involve gene transfer, and can sometimes last the lifetime of the plant. The agricultural and landscape industries would benefit from the registration of a wider variety of effective SAR elicitors.
-Marion Murray, IPM Project Leader