Canker resistance: lesson from kumquat By Naveen Kumar, Bob Ebel The development of Asiatic citrus Throughout their evolution, plants and P.D. Roberts canker in kumquat leaves produced have developed many defense mecha- anthomonas citri pv. citri (Xcc) localized yellowing (5 DAI) or necro- nisms against pathogens. One of the is the causal agent of one of sis (9-12 DAI) that was restricted to most characteristic features associated the most serious citrus diseases the actual site of inoculation 7-12 DAI with disease resistance against entry X (Fig. 2). of a pathogen is the production of worldwide, Asiatic citrus canker. In the United States, Florida experienced In contrast, grapefruit epidermis hydrogen peroxide (H2O2). Hydrogen three major outbreaks of Asiatic citrus became raised (5 DAI), spongy (5 peroxide is toxic to both plant and canker in 1910, 1984 and 1995, and it DAI) and ruptured from 7 to 8 DAI. pathogen and thus restricts the spread is a constant threat to the $9 billion On 12 DAI, the epidermis of grape- by directly killing the pathogen and citrus industry. fruit was thickened, corky, and turned the infected plant tissue. Hydrogen Citrus genotypes can be classified brown on the upper side of the leaves. peroxide concentrations in Xcc-in- into four broad classes based on sus- Disease development and popula- fected kumquat and grapefruit leaves ceptibility to canker. First, the highly- tion dynamics studies have shown that were different. Kumquat produces susceptible commercial genotypes are kumquat demonstrated both disease more than three times the amount of Key lime, grapefruit and sweet lime. resistance and disease avoidance mech- H2O2 that grapefruit produces. Higher Second, moderately-susceptible geno- anisms. Disease resistance can be ex- concentrations of H2O2 seem to be as- types include sweet oranges. Third, the plained on the basis of a decline in Xcc sociated with a decline in Xcc popula- least susceptible class includes manda- populations at 4 DAI. Disease avoid- tions in kumquat. rin, citron and Persian lime. The fourth ance is characterized by the abscission Our goal is to artificially enhance class is highly resistant and includes of Xcc-infected leaves from 7 to 12 the concentrations of H2O2 in Xcc-com- close citrus relatives such as calamon- DAI. Abscission of infected leaves promised grapefruit through an under- din, Ichang papeda and kumquat. not only reduces bacterial load on the standing of the mechanisms of canker Kumquats belong to the genus plant, but may also protect adjoining resistance in kumquat. We are using Fortunella. A kumquat tree is small, leaves and plants from further disease eco-friendly chemical compounds to evergreen and bears small dissemination by wind and rain. augment the concentration of H2O2 in edible fruits. Although kum- susceptible varieties quats are in a different genus of citrus. than commercial citrus, plant We also believe breeders have used kumquats kumquat is an indis- because of their canker resis- pensable source of tance and sexual compatibility canker resistance and with citrus. needs to be evaluated The citrus physiology carefully at morpho- laboratory at the Southwest logical, physiologi- Florida Research and Educa- cal, biochemical and tion Center is exploring the genetic levels to design biochemical basis of kumquat suitable strategies for resistance to citrus canker, canker control in sus- which may be used to de- ceptible citrus crops. velop canker-resistant citrus genotypes. Following artificial Naveen Kumar, Bob Ebel and P.D. Roberts work at inoculation of Xcc in kum- the University of Florida- quat and grapefruit leaves, 75 IFAS’s Southwest Florida percent decline in bacterial Research and Education population was observed in Center in Immokalee. kumquat leaves four days after inoculation (DAI) (Fig. 1). However, a Fig. 1 (above). 1,000-fold increase in Xcc populations Bacterial popula- was observed in canker-susceptible tion (colony form- -2 grapefruit. Similar observations were ing units cm ) also observed in kumquat by others. in kumquat and Canker development in citrus is grapefruit leaves governed by a characteristic sequence of events. The first visual symptom Fig. 2 (right). Vi- after artificial inoculation with the sual symptoms of bacterium is water soaking followed canker develop- by raised and ruptured epidermis at ment in kumquat later stages of canker development. and grapefruit Epidermal rupturing is of ecologi- leaves. The num- cal significance since Xcc infection bers at the bot- induces cell division in plant cells tom represent the causing a rupture of epidermis. This number of days provides a means of dispersal of Xcc after inoculation via action of wind and rain. (DAI). 22 CITRUS INDUSTRY • February 2013.