Mucor rot - An emerging postharvest disease of mandarin fruit caused by Mucor piriformis and other Mucor spp. in California

Authors: Saito, Seiya; MICHAILIDES, T - Kearney Agricultural Center; Xiao, Chang-Lin

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/26/2016
Publication Date: 5/6/2016
Citation: Saito, S., Michailides, T.J., Xiao, C. 2016. Mucor rot - An emerging postharvest disease of mandarin fruit caused by Mucor piriformis and other Mucor spp. in California. Plant Disease. 100:1054-1063. doi: 10.1094/PDIS-10-15-1173-RE.

Interpretive Summary: In recent years, an emerging, undescribed postharvest fruit rot disease was observed on mandarin fruit after extended storage in California. In 2015, we collected decayed mandarin fruit from three citrus packinghouses in the Central Valley of California and identified this disease as Mucor rot caused by Mucor spp. Percentage of Mucor rot in the total decayed fruit varied greatly among 15 grower lots sampled, ranging from 0 to 93.1% with an average of 36.3%. In total, 197 isolates of Mucor spp. were obtained from decayed mandarin fruit and identified based on DNA fingerprinting and morphological characteristics. Of the 197 isolates, 182 (92.4%) were identified as M. piriformis, seven (3.6%) were M. circinelloides (six M. circinelloides f. lusitanicus and one M. circinelloides f. circinelloides), four (2%) were M. racemosus, three (1.5%) were M. hiemalis, and one (0.5%) was M. mucedo. All five Mucor spp. caused decay on mandarin fruit inoculated with the fungi, and the lesion size caused by M. piriformis was significantly larger than those caused by other species at both 5 and 20°C. Our results indicated that Mucor rot in mandarins in California is caused by a Mucor species complex consisting of M. piriformis, M. circinelloides, M. racemosus, M. hiemalis, and M. mucedo, with M. piriformis being the dominant and most virulent species. Previously, M. racemosus was reported on citrus. This is the first report of Mucor rot in citrus caused by M. piriformis, M. circinelloides, M. hiemalis and M. mucedo.

Technical Abstract: In recent years, an emerging, undescribed postharvest fruit rot disease was observed on mandarin fruit after extended storage in California. We collected decayed mandarin fruit from three citrus packinghouses in the Central Valley of California in 2015 and identified this disease as Mucor rot caused by Mucor spp. Percentage of Mucor rot in the total decayed fruit varied greatly among 15 grower lots sampled, ranging from 0 to 93.1% with an average of 36.3%. In total, 197 isolates of Mucor spp. were obtained from decayed mandarin fruit and identified based on ITS sequence and morphological characteristics. Of the 197 isolates, 182 (92.4%) were identified as M. piriformis, seven (3.6%) were M. circinelloides (six M. circinelloides f. lusitanicus and one M. circinelloides f. circinelloides), four (2%) were M. racemosus, three (1.5%) were M. hiemalis, and one (0.5%) was M. mucedo. All species grew at 0 and 5°C except that M. circinelloides did not grow at 0°C. Mycelial growth was arrested at 27°C for M. piriformis, 35°C for M. racemosus, M. circinelloides f. lusitanicus, M. hiemalis and M. mucedo, and 37°C for M. circinelloides f. circinelloides. Optimal mycelial growth occurred at 20°C for M. piriformis and M. mucedo, 25°C for M. racemosus and M. hiemalis, 27°C for M. circinelloides f. lusitanicus, and 30°C for M. circinelloides f. circinelloides. At 5 and 20°C, M. piriformis grew significantly faster than the other four species, and M. mucedo was the most slow-growing one among the five species. Sporangiospores of M. piriformis, M. racemosus, and M. hiemalis germinated at both 5 and 20°C. M. circinelloides germinated at 20°C but did not germinate at 5°C after incubation for 48 h. All five Mucor spp. caused decay on mandarin fruit inoculated with the fungi, and the lesion size caused by M. piriformis was significantly larger than those caused by other species at both 5 and 20°C. Our results indicated that Mucor rot in mandarins in California is caused by a Mucor species complex consisting of M. piriformis, M. circinelloides, M. racemosus, M. hiemalis, and M. mucedo, with M. piriformis being the dominant and most virulent species. Previously, M. racemosus was reported on citrus. This is the first report of Mucor rot in citrus caused by M. piriformis, M. circinelloides, M. hiemalis and M. mucedo.