Causes of Leaf Yellowing Symptoms in Muskmelons and Cultural Control Practices

Abstract

The leaf yellowing symptoms (LYS) of muskmelon (Cucumis melo L.) are a new epidemic that has spread throughout cultivation regions. The objective of this study was to determine the causal factors of LYS, to characterize symptoms, and to develop cultural control practices. In the first chapter, electron microscopic observation and reverse transcription-polymerase chain reaction (RT-PCR) assay revealed that all LYS affected plant samples were free of major melon viruses including CMV, MNSV, CGMMV, SqMV, WMV, KGMMV, PRSV, and ZYMV. Furthermore, the next-generation sequencing assay showed the LYS-affected plant samples contained Cucurbit aphid-borne yellows virus (CABYV), a member of the genus Polerovirus in the family Luteoviridae. The presence of CABYV in these leaf samples was also confirmed by RT-PCR using the CABYV-specific primers. The CABYV infection in muskmelon has not been reported in Korea until now. The complete genome sequences of 22 isolates of CABYV collected in two years (2013-2014) were determined and analyzed comparatively along with previously reported CABYV genome sequences. They contained 5,680-5,684 nucleotides in length and encoded six open-reading frames which were separated into two regions by a non-coding internal region of 199 nucleotides. Their genomic organizations were typical for a member of the genus Polerovirus. When phylogenetic relationship was analyzed with four known groups of CABYV (Asian, Mediterranean, Taiwanese, and R), Korean CABYV isolates were clustered with the Asian group over 94% nucleotide sequence identity. The nucleotide sequence identities of the Korean CABYV isolates with other groups were 87-89% with Mediterranean, 88% with Taiwanese, 81-84% with CABYV-R group, and 72% with Melon aphid-borne yellows virus. This study confirmed that LYS in muskmelon was not merely a physiological disorder, but a viral disease caused by CABYV which was transmitted by aphids. In the second chapter, the physiological and morphological influences of the CABYV-infected muskmelon were characterized. Concentrations of glucose, fructose, and sucrose were greater in CABYV-infected leaves than in uninfected leaves. Sugar contents in stem, fruit, and root tissues were slightly lower in CABYV infected plants than in non-infected plants. The scanning electron microscope observation revealed that the sieve tubes of the leaf vessels were closed more frequently in the CABYV-infected plants than in normal plants. The blockage of the sieve tubes could have restricted the translocation of free sugars from the leaf tissues to other organs in CABYV-infected plants that always produced smaller root masses. The average photosynthetic activity (4.09 µmol·m-2·s-1) was measured by chlorophyll fluorescence yield in the leaves of CABYV-infected plants and it was about one third of that (12.4 µmol·m-2·s-1) measured in normal plants. The average root function (0.28 mg·g-1) of CABYV-infected plants was about the half of that (0.48 mg·g-1) observed in uninfected plants. Cytological observations revealed that the infected and uninfected leaves showed similar morphological characteristics in palisade parenchyma and mesophyll spongy cells of leaves. However, leaf cells of the infected plants contained more starch granules compared to those of uninfected plants. And in the third chapter, various cultural practices were applied as means for reducing LYS in muskmelons. Root pruning before transplanting increased percent incidents of LYS lowering fruit quality, while showing little difference in plant growth. When the aged seedlings were transplanted and cultured, the fruit weight was reduced by LYS development, while plant growth, photosynthesis, and root activity were not significantly different compared to those of muskmelon plants grown from young seedlings. To minimize the occurrence of LYS, seedlings to be transplanted should have less than three fully-expanded leaves at transplanting. Minimizing the number of fruits per plant lowered the incidence of LYS, with 2.4, 6.7, and 13.3% of plants developing the symptoms. As number of leaves left on the vine increased, net photosynthesis and biomass yield increased, while showing less LYS and better fruit quality except the net index of the fruit skin. More than 25 leaves should be maintained on each plant to negate the deterioration of fruit quality influenced by LYS. The incidence of LYS development was reduced by increasing the number of leaves left on the vine above the fruit-bearing nodes.