Several Fusarium species cause disease in onion (Allium cepa), resulting in significant yield losses. In this study, isolates of Fusarium oxysporum and Fusarium proliferatum from diseased and symptomless mature onion bulbs were tested for pathogenicity on onion seedlings and mature bulbs. For F. oxysporum isolates, the outcome of the test on mature bulbs correlated well with the symptom status of the original bulb, whereas for F. proliferatum, such a correlation was not seen, suggesting a different mechanism of symptom development. Those F. oxysporum isolates that carried the Secreted In Xylem (SIX) genes, CRX genes and C5 gene were significantly more aggressive on both seedlings and mature bulbs. However, the SIX-negative F. oxysporum isolates also significantly reduced the emergence of onion seedlings, and thus a division into pathogenic and nonpathogenic F. oxysporum isolates was not obvious. On mature bulbs, those SIX-negative isolates that carried the gene CRX2 were significantly more virulent than those without CRX2. The F. proliferatum isolates were all pathogenic to onion and carried one SIX2 gene homologue, SIX2-1. The majority of F. proliferatum isolates also carried another homologue, SIX2-2. Several isolates negative for SIX2-2 had another potential virulence gene, CRX2, suggesting that these two genes could encode overlapping functions. The SIX2-2 sequence was identical in all those isolates harbouring it, while SIX2-1 showed variation that agreed with the phylogeny based on the translation elongation factor 1-α sequence. This suggests that the two SIX2 genes of F. proliferatum might have been acquired separately.

中文翻译：

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无症状洋葱（Allium cepa）和镰刀菌基腐洋葱中分离出的尖孢镰刀菌和增殖镰刀菌的致病性

几种镰刀菌属物种会引起洋葱（Allium cepa）疾病，导致严重的产量损失。在这项研究中，测试了从患病和无症状的成熟洋葱球茎中分离的尖孢镰刀菌和增殖镰刀菌对洋葱幼苗和成熟球茎的致病性。对于F . 尖孢菌分离株，成熟球茎的测试结果与原始球茎的症状状态密切相关，而对于尖孢菌。proliferatum，没有发现这种相关性，这表明症状发展的机制不同。那些F。 _ 尖孢菌分离株携带木质部分泌( SIX ) 基因、CRX基因和C5基因对幼苗和成熟球茎均显着更具攻击性。然而，六负F。尖孢镰刀菌分离株还显着减少了洋葱幼苗的出现，从而将其分为致病性和非致病性尖孢镰刀菌。尖孢菌分离株不明显。在成熟的球茎上，携带CRX2基因的那些6阴性分离株比没有CRX2的那些分离株的毒力明显更高。F . _ 增殖体分离株均对洋葱致病，并携带一个SIX2基因同源物SIX2-1。大多数F . proliferatum分离株还携带另一种同源物SIX2-2。SIX2-2阴性的几个分离株具有另一个潜在的毒力基因CRX2，表明这两个基因可能编码重叠的功能。SIX2-2序列在所有含有它的分离株中都是相同的，而SIX2-1显示出与基于翻译延伸因子 1-α序列的系统发育一致的变异。这表明F的两个SIX2基因。proliferatum可能是单独获得的。