Short-term CO2 exposure at curing temperature to control postharvest green mold of mandarins

Alternatives to synthetic fungicides are needed worldwide to control citrus postharvest diseases. Curing of citrus fruit at 30-37 degrees C and 90-98% RH for 65-72h has been repeatedly reported as an effective method to control green mold caused by Penicillium digitatum. However, commercial adoption of this practice is limited because of the length of treatment and risks of negative impact on fruit quality. Exposure to CO2-enriched atmospheres is known to provide fungistatic effects and, in some cases, to induce resistance in fresh fruit against major postharvest pathogens. To improve curing treatments against citrus postharvest green mold, work is in progress to determine if short treatments with CO2 at curing temperature can control established infections and/or induce fruit resistance. Mandarins 'Nadorcott' and 'Ortanique' were artificially inoculated with P digitatum and 24h later, were exposed to air (control) or 15, 30 and 50 kPa CO2 at 20 or 33 degrees C for 8 or 24h. Number of decayed fruit was counted after 4 and 7 d incubation at 20 degrees C. To assess potential induction of resistance and time of induction, the same treatments were applied to another set of non-inoculated fruit. Treated fruit were kept at 20 degrees C for 1, 2 or 5 d, at which time mandarins were wound inoculated with the pathogen. Lesion diameter was then measured after 3 and 6 d at 20 degrees C. For both cultivars, only treatments at 33 degrees C for 24h reduced the incidence of disease significantly after 4 d incubation, with 15 kPa CO2 slightly superior to other gas concentrations used. Regardless of time between treatment and inoculation and incubation period, all treatments were ineffective in reducing disease severity and, therefore, in inducing any kind of fruit resistance. It is planned to evaluate longer exposure times (maximum of 48h) to 15 kPa CO2 at 33 degrees C.