Some diseases and pests affecting forest trees Gösta Eriksson and David Clapham 1 Some diseases and pests affecting forest trees Gösta Eriksson David Clapham ISBN 978-91-576-9752-3 Cover photo. Some of the 10% ash dieback resistant Fraxinus excelsior trees in a central Lithuanian progeny trial at Kedainiai, Photograph Alfas Pliura. © The authors 2 Some diseases and pests affecting forest trees Objectives The objective of this publication is to present: General information about diseases and pests including some definitions Harmful pests and diseases in some temperate tree species Information on genetic variation in attacks by fungi or insects following spontaneous or artificial inoculation or insect exposure Measures taken to mitigate fungal and insect attacks. This paper does not give a full coverage of the vast ge- netic research related to forest tree pests and diseases. Pathogens affecting poplars and eucalypts are not treated. They deserve separate treatments. Picture 1. A Pinus taeda tree severely infected with fusi- form rust, which attacks both stems and branches. Photo- graph Gösta Eriksson. Some important forest tree diseases and pests species. In this chapter we illustrate some examples from Conifers genetic studies on important insect herbivores. Dendroc- tonus ponderosae is a small beetle (mountain pine beet- Disease resistance is of utmost importance in several le) that causes severe damage in various pine species in conifers. Fusiform rust, Cronartium quercuum, causes western North America from Mexico to British Colum- great losses for forest owners in South Eastern USA bia. In British Columbia alone, 16 million out of 55 mil- owing to attacks on the commercially important pine spe- lion hectares were destroyed by the mountain pine beetle cies, Pinus elliottii and P. taeda. The symptoms of fusi- (B.C. Forest service 2012). form rust are spindle-shaped galls on stems or branches Pissodes strobi, the white pine weevil, is one of the (Picture 1). During early spring, bright yellow aecial most harmful pests on Picea sitchensis, P. engelmanni, pustules might be observed (aecial = a cuplike structure and P. glauca in western North America. The eggs are of some rust fungi that contains chains of aeciospores, deposited in spring in the leader from last year. The lar- pustule = a small often distinctively colored elevation or vae feed from the bark phloem, which mostly kills the spot resembling a blister pimple). Blister rust caused by leader. This in turn results in deformities of the stem. One Cronartium ribicola is another important disease affec- example of the severity of Pissodes strobi attacks on Sit- ting white pine species such as Pinus strobus, P. strobi- ka spruce was given by King et al. (2004), who reported formis, P. lambertiana, and P. monticola. Brunchorstia that only five trees in a 10-ha Sitka spruce plantation had disease on Pinus sylvestris and P. contorta in Scandinavia survived the white pine weevil attack. Results from two caused by Gremmeniella abietina leads to severe damage studies of resistance to this insect will illustrate genetic certain years. Dothistroma septosporum causes serious variation in susceptibility to this harmful insect. damage in Pinus radiata in Australia and New Zealand. In periodic outbreaks the spruce budworm, Christoneura It is sometimes referred to as red band needle blight or fumiferana, defoliates Picea and Abies trees in its distri- Dothistroma blight. The former was attributed to the red bution areas in eastern North America, and causes morta- color of the needles. Initially following infection, the lity and great losses in wood yield during such outbreaks. needles turn yellow and later on the needle tips become reddish brown. Mist and rain spread the disease, as well Deciduous trees as direct contact, with most damage in humid climates. There are numerous insects that are dependent on forest Two Dutch scientists described a dieback in elms in trees for their existence, some of them extremely harm- 1921, which gave rise to the name Dutch elm disease, ful. Genetic studies are sometimes missing even if some DED. This disease was caused by a fungus, Ophiostoma insect species cause serious damage to their host tree ulmi. During the late 1960s a still more virulent fungus, 3 Ophiostoma novo-ulmi, attacked elms. The American In the search for genetic variation in disease resistance elm, Ulmus americana, and the European elm species it is of importance to analyze if the resistance is caused are seriously, and most often, lethally damaged by DED. by active defence in the host tree or if it could be attribu- In Europe the disease is spread by two beetles, Scolytus ted to escape from being exposed to a particular pest or scolytus and S. multistriatus. In addition to the latter spe- pathogen. A false resistance might exist if the pathogen cies, Hylurgopinus rufipes is also a transmitter of DED or pest has not reached the area of observation at the time in America. DED was spread to America somewhat later of assessment of resistance. Thus, such a resistance is than its first appearance in Europe. attributed to avoidance of exposure to biotically harmful The American chestnut, Castanea dentata, was a domi- organisms. In particular, with global warming a faster ex- nating deciduous tree species in forests of eastern USA pansion of pathogens and pests might take place than the until the early 1900 when chestnut blight, caused by the expansion of host species. This means that the host spe- fungus Cryphonectria parasitica, was introduced by im- cies will become exposed to harmful organisms that they ports of the Asian Castanea crenata, which is almost re- were not exposed to earlier. Since there was no selection sistant to this fungus. Since then this serious disease has for resistance before, a high susceptibility is expected un- spread and more or less wiped out C. dentata in Eastern der these conditions. USA. In 2020 almost only young seedlings of this species could be found in its distribution area. There is a great Different phases in the response to a pathogen may be public interest to restore this charismatic species. Great identified. An hypothesis with three steps in defence efforts were, and still are, devoted to restoration of this against Phytophtora attacks of chestnut tree roots was keystone species. presented by Santos et al. (2017): In 1992 dieback in Fraxinus excelsior was detected in 1. Avoidance of attacks by morphological change or Poland, and a few years later it was found in neighbo- antifungal defence around the roots ring Lithuania. This disease is caused by the Asian fungus 2. Recognition of the parasite Hymenoscyphus fraxineus; it is still spreading in Europe 3. Activation of host resistance genes in 2020 and has killed millions of F. excelsior trees. The This model may apply to many other pathogens and not seriousness of this disease has triggered several forest ge- be limited to Phytophtora attacks. An analogous model neticists to study resistance to it. for insect attacks was presented by Sniezko and Koch The three insect species Lymantria dispar, Popillia (2017): japonica, and Hyphantria cunea are herbivores feeding Traits that deter or repel insect feeding or oviposition from Castanea dentata. Lymantria originates from Latin Traits that reduce insect fitness and/or survival and means destroyer. The moth Hyphantria cunea is na- Tolerance, which means that hosts withstand or recover tive to North America, ranging from Canada to Mexico. from insect attacks It is one of the few insect pests introduced from America Examples indicate that there is a low frequency of trees into other continents. Popillia japonica in North America that exert an active defence. is a noted pest of about 300 species of plants. The leaves It is worthwhile to consider the question as to whether of the host plants attacked are skeletonized by this insect resistance is polygenically inherited or achieved by one species. or a few major genes. Since the generation times of Dryocosmus kuriphilus is a harmful insect that was ob- forest trees usually vastly exceed those of the pathogens served on Castanea sativa in Europe during the present or pests, it is unlikely that the resistance depends on ma- century. In one provenance trial in Italy the affected per- jor genes. If resistance depended on one or a few major centage of trees increased from a few per cent in 2004 to genes, it could easily be lost by mutations in the patho- around 90% in 2007. gens or pests enabling renewed virulence. Therefore, in species with much longer generation times than their pat- Resistance - tolerance hogen(s), polygenic inheritance is more likely than sing- le-gene resistance. From a breeding perspective durable The severity of diseases or pest attacks may vary, and dif- resistance is aimed at, and this is more easily obtained ferent terms are used to describe the severity, sometimes with polygenic inheritance. The importance of long-time with different definition of the terms used. The following field experiments for identification of durable resistan- definitions were given byWoodcock et al. (2019): ce was stressed by Sniezko et al. (2019): Because trees Complete resistance: no or minimal damage is caused are long‐lived, resistance must be effective for decades by pathogens or pests to centuries to be useful. Resistance needs to be durable, Partial resistance: the effects of attacks are limited stable, and present at a useful level. Field plantings provi- Tolerance: symptoms are displayed, but growth and per de the best opportunity to assess resistance durability and formance are maintained to some extent. stability across a range of environments. 4 Table 1. Examples of relationships between diseases and In Table 1 some spectacular cases of introduced fungal di- host species. These diseases listed have great impacts on seases are listed together with native pathogens.