荷兰榆树病在北达科他州:一个新的看法

这种疾病是由两种密切相关的真菌引起的， Ophiostoma ulmi (Buismann) Nannf. 和 Ophiostoma novo-ulmiBrasier, both of which are transmitted by bark beetles 和 through root grafts. 所有的北美榆树都是易感的, 没有一种榆树能完全抵抗这种疾病. This disease is responsible for the death of the majority of native elm trees in the United States in the past century 和 kills a portion of the remaining elm trees every year.

The three species of bark beetle found in North Dakota that spread DED: (A) native elm bark beetle — Hylurgopinus rufipes(B)较小的欧洲榆树树皮甲虫 Scolytus multistriatus (C)带榆树皮甲虫 Scolytus schevyrewi. (图片来源:迈诺特州立大学Guy Hanley)

Three species of bark beetle (Coleoptera: Curculionidae) attack elm trees in North Dakota. These three species are the native elm bark beetle (Hylurgopinus rufipes Eichoff, 图1), the introduced smaller European elm bark beetle (Scolytus multistriatus Marsham, 图1B) 和 the recently introduced b和ed elm bark beetle (Scolytus schevyrewi Semenov, 图1 c). These species are generally small 和 brown or black, with cylindrical-shaped bodies.

Beetles move into a new area by flying short distances or being moved in human-transported wood, 可能会在暴风雨中被吹进来. All three species of beetle are potential carriers of the fungi that cause DED 和 all occur in North Dakota. 然而, the smaller European elm bark beetle is extremely intolerant of cold temperatures 和 does not survive most North Dakota winters in large numbers. The native elm bark beetle is more common 和 important in the spread of DED in North Dakota. Recently, the numbers of b和ed elm bark beetles trapped in North Dakota have increased. 这种增加对DED的影响尚不确定.

不管是什么物种, 当成虫数量高的时候, DED感染的风险增加.

Symptom development varies depending on whether a tree was infected by the pathogen through beetles or root grafts. 被甲虫感染的树木首先表现出枯萎的症状, curling 和 yellowing of leaves on one or more branches in the upper crown (图2). Large trees may survive only two or three years if the disease progresses rapidly, 或者它们可以存活数年, 随着枯枝的数量逐年增加.

在通过根嫁接感染树木的情况下, DED progresses much more rapidly; wilting often begins in the spring in the lower branches 和 progresses upward, 导致了夏季和秋季的死亡率. 如果有根嫁接, elms adjacent to an infected tree usually show symptoms one or two years after the neighboring tree dies.

The pattern of symptom development also can vary depending on the species of bark beetle spreading the fungus. The three elm bark beetles feed on branches less than 4 inches in diameter, with the smaller European elm bark beetle preferring smaller-diameter branches 和 twigs.

Infections due to the smaller European elm bark beetle result in yellowing 和 wilting of leaves on small branches 和 twigs high in the crown. The yellowing 和 wilting then progress to larger branches as the infection moves downward. Removal of bark from these small branches 和 twigs will reveal brown streaking in the sapwood, 这是DED的特征症状(图3)。.

Infections caused by the native elm bark beetle are initiated further down in the crown due to the beetle’s preference for feeding on larger branches. Symptoms usually appear as the wilting or browning of leaves of an entire branch or segment of a tree. Streaking will be present in the larger branches but not the smaller twigs. These infections begin lower down in the crown; by the time symptoms are noticed, 真菌通常已经扩散到大的支架树枝上.

The different patterns of disease development associated with the different species of bark beetle have important management implications.

Wilting leaves on a flag branch is a characteristic symptom of DED. (Photo courtesy 明尼苏达州自然资源部 Archive, 明尼苏达州自然资源部, Bugwood.org)

一个健康的(A)和(B)感染的美国榆树分支. Brown streaking (B) is a characteristic symptom of Dutch elm disease.
(摄影:Jared LeBoldus, NDSU)

原生榆树树皮甲虫. 当地的榆树树皮甲虫可以成虫或幼虫越冬. 在春天, female bark beetles are attracted to dead 和 dying trees (such as those killed by DED) or to elm logs in firewood piles to deposit eggs. They burrow below the bark 和 create nuptial galleries along which eggs are laid. Eggs hatch into grub-like larvae, which feed by making tunnels (or galleries) in the tree. Feeding galleries typically run with the wood grain 和 at right angles to the original egg gallery, 哪些延伸到木纹上. When larvae are mature, they pupate 和 emerge from the tree as adults in July or August ( 图1).

If the tree was infected with DED, the fungus produces sticky spores in the galleries. 当新一代成年甲虫从树上钻出来的时候, they carry these spores 和 transfer the fungus to neighboring healthy trees as they feed on the inner bark of branches. As winter approaches, beetles burrow into the bark near the base of living trees to overwinter. 第二年春天, 甲虫会出现, 为下一代喂养和产卵, 可能会进一步传播疾病.

在北达科他州, native elm bark beetles typically have one generation per year that overwinter as adults or a partial second generation that overwinter as larvae.

Smaller European Elm Bark Beetle 和 the B和ed Elm Bark Beetle. The biology of the European elm bark beetle 和 the b和ed elm bark beetles differs from the native elm bark beetles by having two or more generations per year. Smaller European elm bark beetles overwinter as mature larvae in protected sites such as logs in the interior of firewood stacks. 带状榆树树皮甲虫能以幼虫越冬, 在被感染的树枝或树上的蛹或成虫, 或者在原木上加上树皮.

在六月和七月, adult beetles emerge 和 maturation feed at twig crotches of healthy trees for seven to 10 days. This feeding activity can result in multiple points of pathogen spore transmission to the tree. After feeding, females will fly to dead or dying elms 和 bore into the bark to lay egg galleries.

Larvae hatch from the eggs, excavate larval galleries 和 pupate when mature. The egg gallery runs with the wood grain, whereas the larval galleries radiate out across the grain. After 35 to 40 days, the second generation of beetles emerges 和 the cycle continues.

根移植. Root grafting occurs when tree roots come into contact with each other 和 fuse together, 形成一个连续的血管系统. The DED fungus will produce spores that can spread from infected to healthy trees through this continuous system. The fungus can kill all the elm trees along the length of an entire street in this fashion.

Several other diseases 和 abiotic disorders may cause wilting symptoms 和 the death of elm trees. To be certain that DED is killing an elm tree, the fungus needs to be isolated from infected tissue. For a small fee, samples can be submitted to the Plant Diagnostic Lab at NDSU for verification. 寄送样品至 NDSU植物诊断实验室,部门. 58102法戈市新州立大学华德龙堂203号植物病理学研究室.

有几个用于DED的管理工具可用. Using them in the appropriate situations 和 combinations is the best way to reduce DED 和 limit its spread.

环境卫生. A year-round communitywide sanitation program is an extremely effective way to slow the spread of DED. Because elm bark beetles breed in dead elm wood with intact bark 和 in dying elm trees, 清除这些甲虫栖息地是至关重要的. 这包括在风暴过后移除受损的树枝, trees weakened by pest attack 和 fresh elm firewood (all species of elm).

处理受感染榆树木材的常用方法包括焚烧, 埋葬, 把榆木削碎或剥皮. 使卫生设施有效运作, identifying newly infected trees continuously throughout the growing season each year 和 removing them promptly (within 2-3 weeks) is essential. Failure to do so may result in a surge in elm mortality in the following years. A strong municipal ordinance that clears the way for communitywide sanitation on public 和 private property also is essential to this effort.

修剪. If infections are identified early enough, they can be removed by pruning. At least 8 to 10 feet of wood that is free of vascular discoloration needs to be removed below the symptomatic branches. 修剪应在10月至次年3月进行. 这些修剪过的树枝应立即销毁.

Remember to disinfect pruning tools with a 15-20 percent household bleach solution after exposure to DED-infected trees. 由于不确定修剪掉所有受感染的木材, trees pruned in an attempt to remove DED-infected branches should be monitored closely for the development of symptoms below the pruned area.

切断根嫁接. To limit the spread of DED through root grafts, the area around infected elm trees can be trenched. 如果不到25%的树冠被DED损坏, 然后切断根移植物是一个可行的治疗选择. These trenches should be 36 to 40 inches deep 和 completely encircle infected elm trees. 壕沟可以立即填土, 和 any infected trees should be removed as part of sanitation efforts.

如果超过25%的树冠被DED损坏, 真菌很可能已经进入根部了. 在这种情况下，根修剪不能挽救邻近的树木.
注意:挖沟前, 呼叫北达科他州一号电话, (800) 795-0555 or 811, 确保所有地下管线都有清晰的标志.

杀真菌剂. Fungicide injection may prevent the fungus that causes DED from infecting healthy elms. Two active ingredients, thiabendazole 和 propiconazole, are registered for controlling the fungus. These chemicals need to be injected directly into the tree root flares by a trained tree care professional.

Fungicide injections are labeled for preventative 和 therapeutic treatment but are less effective if the tree already is infected.

杀菌剂不能控制移植物感染. 杀菌剂对DED的保护可以持续三年. Note that while fungicide injection is extremely effective at preventing DED infection, repeated injections can cause irreversible damage to trees 和 phytotoxicity can occur under certain conditions. 杀真菌剂 need to be used in combination with sanitation to be effective in reducing DED.

抗病品种的. 没有一种榆树能完全抵抗DED. 然而, a large amount of variation occurs among species of elm trees. 例如，美国榆树(榆属美国 L.)最易受影响，而西伯利亚榆树(榆属pumila L.)的抵抗力最强. 许多抗d的美国榆树品种, 还有外来的榆树品种和杂交品种, 都可以在市场上买到. 联系 your local county 扩展 agent or arborist for a list of the current DED-resistant cultivars.

杀虫剂控制. Insecticides are not recommended as a preventative treatment to control adult beetles from transmitting DED spores to uninfested trees, since multiple applications would be needed for the prolonged flight periods 和 various emergence patterns of different species of elm bark beetles. 上面提到的其他管理技巧, 如卫生和杀菌剂注射, 如何更有效及更具成本效益地控制DED.