Department of Postharvest Science, ARO, Volcani Center, Bet Dagan, Israel.
Phytopathology. 2011 Apr;101(4):436-44. doi: 10.1094/PHYTO-09-10-0251.
Israeli farmers export 250,000 tons of potato tubers annually, ≈40,000 tons of which are harvested early, before skin set. In recent years, there has been an increase in the occurrence of dark skin spots on early-harvested potato tubers ('Nicola') packed in large bags containing peat to retain moisture. The irregular necrotic spots form during storage and overseas transport. Characterization of the conditions required for symptom development indicated that bag temperature after packing is 11 to 13°C and it reaches the target temperature (8°C) only 25 days postharvest. This slow decrease in temperature may promote the establishment of pathogen infection. Isolates from typical lesions were identified as Rhizoctonia spp., and Koch's postulates were completed with 25 isolates by artificial inoculation performed at 13 to 14°C. Phylogenetic analysis, using the internal transcribed spacer sequences (ITS1 and ITS2) of rDNA genes, assigned three isolates to anastomosis group 3 of Rhizoctonia solani. Inoculation of wounded tubers with mycelium of these R. solani isolates resulted in an oversuberization response in the infected area. With isolate Rh17 of R. solani, expression of the suberin biosynthesis-related genes StKCS6 and CYP86A33 increased 6.8- and 3.4-fold, respectively, 24 h postinoculation, followed by a 2.9-fold increase in POP_A, a gene associated with wound-induced suberization, expression 48 h postinoculation, compared with the noninoculated tubers. We suggest that postharvest dark spot disease is an oversuberization response to R. solani of AG-3 infection that occurs prior to tuber skin set.
以色列农民每年出口 25 万吨土豆块茎，其中约有 4 万吨是在土豆块茎的表皮形成之前提前收获的。近年来，用含有泥炭以保持水分的大袋包装提前收获的土豆块茎（“Nicola”）时，经常出现表皮黑斑。这些不规则的坏死斑点是在储存和海外运输过程中形成的。对症状发展所需条件的特征描述表明，包装后袋内温度为 11 到 13°C，收获后 25 天才能达到目标温度（8°C）。这种缓慢的降温可能会促进病原菌感染的建立。从典型病变中分离出的菌株被鉴定为立枯丝核菌（Rhizoctonia spp.），通过在 13 到 14°C 下进行人工接种，完成了 25 个分离株的柯赫氏假设。使用 rDNA 基因的内部转录间隔区序列（ITS1 和 ITS2）进行的系统发育分析，将三个分离株分配到立枯丝核菌的 3 号吻合群。将这些立枯丝核菌分离物的菌丝体接种到受伤的块茎上，会导致受感染区域过度角质化。用立枯丝核菌 Rh17 菌株接种后，24 小时后与角质化生物合成相关的基因 StKCS6 和 CYP86A33 的表达分别增加了 6.8 倍和 3.4 倍，随后与伤口诱导角质化相关的基因 POP_A 的表达增加了 2.9 倍，接种后 48 小时与未接种的块茎相比。我们认为，采后黑斑病是在块茎表皮形成之前，AG-3 感染立枯丝核菌引起的过度角质化反应。
