The Role of Plant Nutrients in Disease Development with Emphasis on Citrus and Huanglongbing

Mineral nutrients are important for the growth and development of plants and microorganisms, and are important factors in plant–disease interactions. How each nutrient affects a plant’s response to disease is unique to each plant–disease complex, and in general, nutrient–pathogen interactions are not well understood. Plant nutrients may affect disease susceptibility through plant metabolic changes, thereby creating a more favorable environment for disease development. For example, calcium defi ciency can lead to membrane leakage of sugars, amino acids, and other low-molecular weight compounds that then become available for pathogen use. Many nutrient metals at elevated concentrations have broad antibacterial properties so pathogens that directly or indirectly reduce these plant nutrients may have an advantage. Relatively little is known about the changes in plant nutrition associated with huanglongbing (HLB) despite its leaf symptoms often being characterized as “nutrient defi ciency-like.” Recent analyses comparing symptomatic (blotchy mottle) and asymptomatic leaves from HLB-infected trees and leaves from healthy trees have shown that HLB increased K while Mg, Ca, and B decreased. The micronutrients Zn and Mn, whose defi ciency symptoms are commonly seen on HLB-infected trees, were not actually defi cient in HLB-infected samples when the dry mass of the samples was corrected for the large amounts of starch accumulation caused by HLB. It remains to be seen whether remedial foliar applications of these or other nutrients can reduce the effects of HLB. Here we provide a brief review on plant nutrition and disease susceptibility, with an emphasis on K, Ca, Mg, and B, and discuss the changes in these nutrients associated with HLB infection in citrus.