Field Guide for the Identification of Damage on Woody Sentinel Plants

Alain Alain, Michelle Cleary, Iryna Matsiakh, René Eschen

• Publisher: CAB International
• Year: 2017
• ISBN:
  9781789249590
• Topic: Technology & Engineering
• Subtopic: Forestry
• Index: Technology & Engineering

This book is a heavily-illustrated, internationally applicable, practical guide for the identification of likely causal agents of damage to trees and woody shrubs. It is intended for use in sentinel plantings - a new tool to identify pests in the country of origin, used to inform pest risk analysis and risk mitigation measures - where agents often may not be known and only damage visible.Field Guide for the Identification of Damage on Woody Sentinel Plants: - Aids the identification of the type of agent that may have caused observed damage, including pathogens, invertebrates and abiotic factors.- Explains how to take and preserve samples and how to proceed to obtain a more definitive identification of pests.- Includes a general damage chapter in addition to specific chapters on damage to leaves, buds and shoots, roots, trunk, and flowers, fruits and seeds.- Contains 800 full colour, high quality photographs to aid analysis.This is an essential guide for plant health professionals, including inspectors for plant protection organizations, foresters and nursery managers, in addition to students of forest entomology and pathology.

1
Introduction

R. Eschen

Woody plants, whether planted for economic, environmental or ornamental purposes, are valued resources in all countries. Damage due to pests (including invertebrates and pathogens) can be costly, in terms of both direct loss of crops and control tactics to minimise the impact of pests. The most cost-effective option is either prevention of introduction or early detection and eradication of new species before they become widespread or problematic. Many pests are only detected years after establishment however, which makes eradication nearly impossible, particularly in the case of pathogens. The example of the successful eradication of the fungus Fusarium circinatum on pine trees in Italy illustrates that eradication is possible. Such intervention is easier in nurseries and semi-natural environments, but efforts can be very expensive or impossible if the pest has spread before being detected.

Efforts to protect trees from pests often depend on knowledge of the identity and ecology of the pests, as well as their potential impact. This is particularly the case with new pests that are alien (i.e. non-native) to a country, which may be less well studied or even unknown. Alien pests are often introduced accidentally with the intercontinental trade, in particular the trade of living plants. At present, a large number of the newly introduced species are “emerging” species, i.e. species that have not previously been introduced outside their native range (Seebens et al., submitted). They are usually not on any list of potential invaders. Authorities in importing countries can respond to the threat of alien species introductions by taking measures to prevent an introduction. These measures may be implemented in the exporting country to avoid transport, or the importing country tries to detect them at the border or soon after introduction. Hence, it is beneficial, if not necessary, to know the identity of the new emerging alien species to be targeted by such measures.

Sentinel planting concept

In order to know what organisms may be associated with a tree species of interest, and what damage they may do if introduced to a new country, it is possible to look at trees in the countries where trees are imported from in so-called sentinel plantings. Such plantings can be arboreta, botanical gardens or nurseries (planted for this purpose or not: Fig. 1.1.). The studied tree species can be native to the importing country or native to the exporting country and this difference affects the interpretation of the results. Pests found on commonly traded tree species native to the exporting country may indicate that these pests are likely to be shipped on exported plants. By contrast, looking at tree species that are exotic to the exporting country may not provide indications about the likelihood of introduction, but is particularly relevant for the quantification of damage, since the suitability of native species as hosts for introduced pests and their potential impact are almost impossible to predict. Another factor that may influence which pests are found is the age of the trees: on young trees in nurseries foliar pests may be more common than on older trees in arboreta and botanical gardens, which may host more pests of woody organs.

Figure 1.1. Sentinel planting with Chinese trees close to Beijing, China, and people inspecting a tree in the planting for the presence of pests (RE). Inspection of European trees planted in Fuyang, China, for detection of pests and damage potential (AR). Collections in botanical gardens often include exotic trees (Russian Far East, NK).

Studies have shown that sentinel plantings are a very powerful tool to identify organisms associated with tree species. Many of the detected species are unknown associates with the studied tree species or even unknown to science. For example, the vast majority of the insects and fungal pathogens found on European oak trees planted in an experimental sentinel nursery in China could not be identified to species and of those that were identified, most represented new records of those species on the planted tree species (Roques et al. 2015, Vettraino et al. 2015). The knowledge gathered from such sentinel planting examples can be used to inform regulatory bodies in importing countries to be able to perform pest risk analyses on these pests and decide, if necessary, on measures to mitigate the risk of introducing them. Such measures may include restrictions on the season of import, the origin of the imported plants or chemical or non-chemical treatments.

A guide to identify the most likely causes of damage observed on sentinel plants

This guide is intended as an aid for managers of sentinel plantings, botanical gardens or arboreta, as well as phytosanitary inspectors, who may have knowledge of common pests and diseases of woody plants, but may not know the likely cause of damage that they have not encountered before, for example, because it is a new type of harmful organism or a new type of damage on a known tree species. It is also possible that they are unable to distinguish between damage due to abiotic and biotic factors and between symptoms caused by pathogens and insects. This field guide is the fruit of efforts of a large group of scientists from many countries, both entomologists and pathologists, brought together through COST Action Global Warning (https://www.ibles.pl/cost/). These experts have shared their knowledge and pictures to help the user identify what may cause the damage they see on their woody plants.

When inspecting woody plants for pests, it is possible that the first or only thing to see is damage or disease symptoms. In many cases, the causal agent may not be present any more or not visible, but it is nevertheless important to be able to determine what caused the damage in order to determine whether it is due to abiotic conditions or a pest or disease. The aim of this book is to provide a tentative identification of relatively broad groups of organisms and not definitive identification of the causal agents, which requires more in-depth knowledge than we expect most users to have. Hence, the pictures in this guide need to be regarded as typical examples of the described symptoms. The next Chapter explains how to use the guide, followed by keys (for different organs of conifer and broadleaf species) to guide the user to the relevant sections of the book, based on the questions entomologists and pathologists ask themselves when looking at a damaged or diseased tree in a fashion similar to how a doctor interrogates a patient to arrive at a diagnosis. The largest part of the book is devoted to the description and illustration of damage types and typical causes of the observed damage. The last Chapters provide instructions for taking and preserving samples for further identification by an expert, notification of relevant authorities, and a glossary.

References

Roques, A., Fan, J., Courtial, B., Zhang, Y., Yart, A., Auger-Rozenberg, M.-A., Denux, O., Kenis, M., Baker, R. and Sun, J. (2015) Planting sentinel European trees in eastern Asia as a novel method to identify potential insect pest invaders. PLoS ONE 10, e0120864.

Seebens H., Blackburn T.M., Dyer E.E., Genovesi P., Hulme P.E. et al. (submitted) Historical dynamics of alien species, emerging alien species and the pool of potential new invaders, Nature Ecology and Evolution.

Vettraino, A., Roques, A., Yart, A., Fan, J., Sun, J. and Vannini, A. (2015) Sentinel trees as a tool to forecast invasions of alien plant pathogens. PLoS ONE 10, e0120571.

2
Field diagnosis of damaging agents of woody plants

A.C. Moreira, H. Bragança, C. Boavida and V. Talgø

Introduction

The observation and evaluation of trees or shrubs with symptoms is the first step towards a diagnosis. Several damaging agents can give similar symptoms and because tree damage is often due to a combination of causes, field diagnosis is often complex. An overview of the system will be needed to assess the extent of the problem. For example, the symptom distribution pattern of affected trees is important to determine whether an observed pattern of damage is localised or widespread, which may be indicative of the problem being abiotic (e.g., soil flooding/drought) or biotic, caused by an arthropod pest (insect, mite) or by a pathogen) (Fig. 2.1.).

Fig. 2.1. Cork oak (Quercus suber) killed by Phytophthora root rot (Phytophthora cinnamomi) in the south of Portugal (left, ACM) and noble fir (Abies procera) killed by P. cambivora in western Norway (right, VT).

The primary cause of the observed damage is not easy to diagnose without taking a careful and individual approach to the tree. As a first observation for broadleaf trees, the appearance of the crown can be a good indicator of the health status. In particular, determining if the crown presents some degree of defoliation is important, keeping in mind that defoliation may be a reflection of multiple causes. For conifers, chlorotic or necrotic foliage and/or needle cast will help to identify the damaging agent. In any case, the first observations should be assessed further using sound methods. For example, in forests in the Mediterranean region the assessment of the degree of defoliation is based on a scale described by Cadahia et al. (1991) and Roloff (2016), which uses a vitality classification system for urban settings. In most cases, a reliable diagnosis requires specialised analysis to detect or identify the causal agent or to determine other parameters that may be associated with the problem, such as nutrient deficiencies.

Whole plant observation

A diseased tree or seedling experiences deviations from its normal functioning, caused by some type of biotic agent or environmental stress factor. These changes manifest themselves through various symptoms. In addition to the already mentioned defoliation and chlorosis/necrosis, leaf yellowing, wilting of leaves and shoots, branch and stem cankers, root decay and, eventually, tree mortality may occur. Stress factors may include abiotic conditions, such as drought, flooding, strong wind, extreme temperatures, a diversity of unbalanced soil factors (e.g., the amount of organic matter, soil structure, nutrient content, and pH), and poor management practices. Biotic stresses are caused mainly by plant pathogens and arthropod pests, but interactions with abiotic factors can influence disease development and play a decisive role in tree health.

As tree damage progresses, the weakened tree becomes susceptible to secondary invasions by different fungi and insects, which can make it more difficult to clearly diagnose the primary causal agent. Tree dieback and decline symptoms are typically characterised by a gradual loss of tree vigour, i.e. progressive death of young shoots, branches showing discoloration and wilting or loss of foliage, all of which are examples where a complex of pathogens and insects may be involved. The affected trees must be examined carefully by looking for symptoms (such as curling or perforation of the leaves, webbing, galls, presence of exudations or sawdust) and signs (e.g., larvae, insect eggs or fungal fruiting bodies) in order to identify the primary cause of the damage or disease.

In general, many of the signs of biotic, damaging agents are inconspicuous to an untrained eye and therefore observations in the field often require a good hand lens to spot, for example, fruiting structures at a higher magnification, and a knife to cut-open and expose affected plant tissue to reveal characteristic signs of the causal agents or the agents themselves. However, some signs of biotic diseases are specific and easy to detect, like mushrooms growing at the base of the trunks of affected trees.

Observation of individual tree parts

If only a part of the tree appears affected, careful observation of the affected parts may reveal symptoms of disease or pest attacks which can be identified using the keys in this Field Guide. Each of these parts can show a variety of damage morphotypes, which can be related to groups of causal agents, even when the specific causal agent is unknown. By systematically looking at the symptoms, it is possible to exclude some and close in on the most likely causal agent. On the following pages, we present keys for each part of the tree (Table 2.1.), separately for conifer and broadleaf trees, that will guide the user to the sections in the book that provide descriptions of the damage, and some examples of the causal agents that may cause the observed damage.

Table 2.1. The different parts of broadleaf and coniferous woody parts treated in this field guide and the pages where keys for the identification of potential damaging agents can be found.

	Broadleaf woody plants	Coniferous woody plants
Foliage	20	29
Flowers, fruits and seeds	21	30
Twigs, branches and stems	25	34
Buds and shoots	27	36
Roots and collars	28	37

Observation of where and when the damage occurs should direct the user to the appropriate key, i.e. where the cause of the problem can be found. For example, if only individual branches are affected on broadleaf trees, it is likely that the cause can be found on those branches, as opposed to for example affected leaves that can be seen dispersed over the entire crown. Pests and diseases of foliage, buds, shoots and reproductive organs (flowers, developing fruits, seeds) tend to be seasonal and fungi that cause foliar diseases are typically most serious during wet springs or autumn. Cankers on branches or stems may be more easily observed on broad leaf trees when the trees have lost their leaves, or during the season, when the cankers are fruiting. On conifers, so-called flagging (dieback of basal or distal branches) often reveals canker wounds.

If the whole tree is affected, it is possible that it faces a problem with its roots. Root damage is often caused by soil borne organisms which affect the tree’s ability to absorb nutrients and water, thereby leading to drought symptoms and nutrient deficiency. Sometimes this even causes trunk bleeding (resin flow on conifers). Some microbial agents cause “decline and dieback” or “sudden death”. In the latter case, the tree dies suddenly with dry leaves still attached.

Diagnosing a root problem can be particularly difficult because several ...