Grey mold and cottony rot caused by Botrytis cinerea and Pythium aphanidermatum, respectively, were the most serious post-harvest diseases attacking snap bean. Five organic acids, i.e. ascorbic, citric, boric, salicylic and acetic, were tested for controlling both fungal diseases attacking snap bean pods of cvs, Xera and Valentino in vivo. The tested organic acids were sprayed on green bean plants of the two varieties for three times, at blooming stage, and repeated every 10 days as time interval between sprays till 5 days before harvesting, during seasons 2012 and 2013. The harvested snap bean pods were kept in perforated polyethylene consumer bags or in a type of modified atmosphere package (Xtend® films Easy-Tear bags). Valentino pods of control treatment showed higher susceptibility to both diseases compared with Xera pods.
Pre-harvest spray of the tested acids inhibited completely the decay development of naturally infected pods of both snap bean varieties during storage at 7±1°C and 90-95% RH for 18 days expect
acetic acid on cv. Xera at the lower concentration 0.1%, which controlled the both post-harvest diseases with efficacy about 70%. Boric, acetic and ascorbic acids showed minimum decay caused by B. cinerea at the high concentration (1, 0.2, 2%, respectively) compared with those treated with citric and salicylic acids. As for cv. Valentino, citric acid at 2% was the most effective treatment against fungal decay of artificially inoculated snap bean pods with B. cinerea as well as
naturally infected pods. However, boric, citric and ascorbic acids with all tested concentrations inhibited the cottony rot on snap bean pods artificially inoculated with P. aphanidermatum.
Pre-harvest spraying of snap bean with organic acids incited anatomical changes in cuticle and epidermis of pods. In this respect, spraying boric acid on green bean plants cv. Xare was the most
effective treatment in increasing the thickness of epidermis of snap bean pods. The cuticle of Xera pods harvested from sprayed plants with tested organic acids were thicker than the control. Meanwhile, spraying organic acids on green bean plants cv. Valentino did not affect the epidermis layer thickness of pods. While, salicylic, acetic and citric acids sprays increased cuticle layer thickness. However, the anatomical changes either in epidermis and/or cuticle induced by pre-
harvest organic acid sprays was recorded.
Phytoalexins content was detected in naturally and artificially infected snap bean pods of pre-sprayed bean plants with tested organic acids post 72 hr of harvest. The highest concentrations of coumestrol and 6-a-Hydroxyphaseollin were detected in artificially infected Valentino pods with B. cinerea pre-spraying with salicylic acid. Also high levels of coumestrol, kievitone as well as 6-a-Hydroxyphaseollin in snap bean pods cv. Valentino infected with B. cinerea and sprayed with boric acid. The highest concentration of 6-a-Hydroxyphaseollin (5048.63 pg/ kg fresh weight) was recorded in snap bean pods cv. Valentino infected with P. aphanidermatum. Salicylic acid increased
the phaseollidin concentration in both tested varieties artificially infected with P. aphanidermatum than the control. Phaseollin was highly detected in artificially inoculated Xera pods with B. cinerea
than in Valentino pods. |
