褐根病菌無病徵感染草本植物之探討

樹木褐根病由白腐菌 Phellinus noxius 所引起，受感染之樹木根部腐朽、枝葉黃化及小葉化、全株樹勢衰弱，易受風傾倒，為熱帶及亞熱帶地區重要之樹木病害。現行罹病地處理原則各有其優缺點，其中自然復育法係基於褐根病菌無法單獨殘存於土壤中，故只需撿除罹病殘根且避免種植寄主植物，即可達到良好防治效果，然實務上可能因為殘根清除不確實，導致病地之雜草及補植花草仍有接觸病原菌之機會。為瞭解褐根病菌是否能感染草本植物，本研究首先針對台北、桃園、台中及高雄地區，共 31 個褐根病罹病地之草本植物進行採樣調查，結果顯示由 8 種草本植物 (土半夏、兩耳草、雙穗雀稗、竹葉草、貓爪藤、大花咸豐草、升馬唐及台北草) 可分離到褐根病菌，其中以竹葉草之檢出率最高。透過盆栽內人工接種試驗，發現可由魁蒿、竹葉草及台北草之根系檢出褐根病菌，但無法從巴西地毯草、假儉草、玉龍草及百慕達草根系檢出褐根病菌；受感染之魁蒿於 2 週內全株萎凋，屬於高度感病之有病徵草本寄主，而竹葉草及台北草則無論地上部或地下部均無病徵，植株維持翠綠，生長勢旺盛，且僅接種處可分離到褐根病菌，屬於無病徵草本寄主。進一步於水瓊脂培養基上進行草本植物幼苗根組織接種，利用石蠟切片及台盼藍組織染色法可觀察到褐根病菌成功侵染所有受測植物 (竹葉草、韓國草、類地毯草、假儉草、百慕達草及馬蹄金) 之根系，於接種 1 週後菌絲在皮層組織拓殖，2 週後菌絲明顯增多且向中柱蔓延侵染；雖然此接種模式並非完全符合自然條件，但仍顯示於營養缺乏下，褐根病菌可能具有無病徵感染幼嫩草本植物之能力。為瞭解褐根病菌感染草本植物之偏好性，利用溴化乙醯法測定總木質素含量，發現褐根病菌寄主魁蒿、竹葉草及台北草相對其他草坪植物高，然而感病木本寄主茄苳之木質素含量卻不高，因此木質素總量可能並非影響褐根病菌病原性之主要原因。最後，為評估補植對於罹病地復育之效果，將人工接種褐根病菌之細木條埋於有或無植被之土壤中，4 週後發現只有裸土中帶菌木條尚能檢測出褐根病菌，而有植被者均無法分離出褐根病菌。鑑於本研究，罹患褐根病之樹木移除時，除了徹底清除樹頭及殘根外，建議連同病樹周圍草本寄主植物 (如魁蒿、竹葉草、台北草等) 一併去除，之後可補植非寄主之草坪植物 (如巴西地毯草、假儉草、玉龍草及百慕達草等)，以加速殘體內褐根病菌降解及病地復育。

關鍵字

褐根病；無病徵感染；草本植物；木質素；自然復育法

並列摘要

Brown root rot (BRR), caused by the white-rot fungus Phellinus noxius, is an important tree disease in tropical and subtropical areas. P. noxius-infected trees suffer from decayed roots, chlorotic and small leaves, weakening, and are easy to topple over during strong winds. Different strategies for remediation of BRR infection sites have their pros and cons. The natural remediation is based on the fact that P. noxius cannot survive in the soil, so the disease can be well-controlled by removing the residual roots and avoiding the replantation of host plants. However, in practice, the onsite or replanted herbaceous plants may still be exposed to the pathogen due to incomplete removal of the debris. To understand whether P. noxius can infect herbaceous plants, herbaceous plants in 31 infection sites in Taipei, Taoyuan, Taichung, and Kaohsiung were sampled. P. noxius was isolated from eight herbaceous plants, including Typhonium blumei, Paspalum conjugatum, Paspalum distichum, Oplismenus compositus, Macfadyena unguis-cati, Bidens pilosa, Digitaria ciliaris, and Zoysia matrella, among which O. compositus showed the highest detection rate. Pot inoculation assay showed that P. noxius was detectable from the inoculated roots of Artemisia princeps, O. compositus, and Z. matrella, but not from Axonopus compressus, Eremochloa ophiuroides, Ophiopogon japonicus, and Cynodon dactylon. The infected A. princeps were wilted within two weeks post inoculation, suggesting that it is a highly susceptible symptomatic herbaceous host. The inoculated O. compositus and Z. matrella showed no symptoms in their aboveground or underground tissues and remained green and vigorous, and P. noxius could only be isolated from the inoculated sites. These suggested that O. compositus and Z. matrella are asymptomatic herbaceous host of P. noxius. Inoculation of the roots of herbaceous juvenile seedlings was conducted on water agar. Paraffin sectioning and trypan blue staining revealed that P. noxius successfully infected the roots of all tested plants (i.e., O. compositus, Zoysia tenuifolia, Axonopus affinis, E. ophiuroides, C. dactylon, and Dichondra micrantha). P. noxius hyphae colonized the cortex tissues after one week of inoculation, then propagated and invaded into the vascular cylinder after two weeks of inoculation. Although the juvenile seedling inoculation system was not fully natural, the results indicated the potential of asymptomatic infection of P. noxius in young tissues of herbaceous plants under nutrient deficient conditions. To understand the infection preference of P. noxius, the acetyl bromide soluble method was used to analyze the total lignin content. The lignin contents of P. noxius hosts, A. princeps, O. compositus, and Z. matrella, were higher than other turfgrasses. However, the lignin content of the susceptible woody host Bischofia javanica was not high, indicating that the total lignin content may not be the main factor affecting the pathogenicity of P. noxius. Finally, to evaluate the effect of replantation for the remediation of infection sites, P. noxius-inoculated wood sticks were buried in soil with or without vegetation. After 4 weeks of inoculation, P. noxius could only be detected from the inoculated wood sticks buried in bare soil but not the soil with vegetation. According to the findings of this study, in addition to thorough removal of diseased tree stump and residual roots, it is also recommended to remove the herbaceous host plants (such as A. princeps, O. compositus and Z. matrella) around the diseased trees. Afterwards, nonhost turfgrasses (such as A. compressus, E. ophiuroides, O. japonicus, and C. dactylon) can be replanted to accelerate the degradation of P. noxius in the debris and the remediation of infection sites.