Arboriculture & Urban Forestry 43(6): November 2017 ress of the fungus (D’Arcy 2000). Lignin and suberin, more resistant to fungal enzymes than cellulose, are also synthesized. A barrier wall, which remains alive, is then formed a distance from the infection (Shigo and Marx 1977). Anatomical studies of susceptible and resistant elm species have looked for correlations between anatomical features and resistance. Early research established a statistical correlation between the length and diameter of xylem vessels and the susceptibility to DED. Vessel length and water conductivity were greater in susceptible elms (Elgersma 1970). In shorter vessels, tyloses more quickly block the vertical movement of the patho- gen, thereby reducing the extent of its movement down a limb (McNabb et al. 1970). More recently, however, conclusions based on such findings have been questioned, since seasonal changes in vessel diameter and changes in vessel dimen- sions with branch age are not always considered and can impact the results in infection studies (Martin et al. 2013). Elms are more susceptible when the season promotes rapid vessel elonga- tion (Santini and Faccoli 2014). Statistical corre- lations have been found between tree age, xylem vessel diameter, and the extent of wilting (Solla et al. 2005). It has been hypothesized that drought resistance, an environmental factor that can also cause cavitation in xylem vessels, might make a drought-resistant elm genotype less vulnerable to DED (Venturas et al. 2014). Using progeny derived from crosses between DED-susceptible and DED-resistant individuals of Ulmus minor, it was noted that susceptible offspring had wider and longer vessels; while xylem structure seemed to influence the spread of the pathogen, it did not prevent cavitation. After studying a series of elm hybrids with differing susceptibility, it was concluded that the structural basis of resistance to cavitation caused by the pathogen may be too multifaceted to be captured by single traits, such as vessel measurements, and therefore, more com- parative work incorporating numerous hydraulic parameters is essential (Martin et al. 2013). It would be simplistic to think that vessel anatomy alone can predict susceptibility to DED. There are DED-tolerant and DED-intolerant American elms with similar anatomy. There are species of trees with vessel anatomy similar to 223 American elm that do not die of DED. Yet, it has been pointed out that to acquire DED the patho- gen must be transmitted by a vector that is specif- ically attracted to elm trees (L. Bernier, personal communication). Other tree genera, therefore, may never be exposed to Ophiostoma. Non-host species (Prunus pensylvanica and Populus bal- samifera) that were mechanically inoculated with the DED pathogen became infected, but ana- tomical studies showed that continuous barrier zones containing suberin and lignin developed, whereas barrier zones were discontinuous in American elm (Rioux and Ouellette 1991). The barrier zones also formed more quickly in Prunus and Populus than in American elm. Re-isolation of the pathogen from a non-host plant (Rioux and Ouellette 1989) implies that host specificity of the DED pathogen may be overstated. Bernier (personal communication) is curious as to why this lack of host specificity is not cited more often, as it suggests the possibility that DED pathogens could jump to non-elm hosts if the fungus was acquired by vectors other than elm bark beetles. Elms are not equally susceptible to contract- ing DED throughout the growing season, and this may be a combination of vessel morphol- ogy and vector behavior and life cycle. In stud- ies of elms that broke bud (“flushed”) earlier than others, the early flushers were more DED resistant (Ghelardini and Santini 2009). This fact may also explain discrepancies in the rank- ing of elms for DED susceptibility, as the tim- ing of resistance tests between research groups is variable, which can affect the interpretation of results or manifestation of symptoms. For example, ‘American Liberty’ clone 503 (Smal- ley et al. 1993), which displays increased DED resistance in field studies, was very sensitive to DED when inoculations were performed in controlled growth chambers after the leaves had fully expanded (Et-Touil If et al. 2005). the cause of DED resistance were fully understood, breeding resistance into susceptible species would be more focused. As with many other pathogens, compartmentalization associ- ated with the synthesis of lignin and suberin is a typical response of a DED-infected tree (Duch- esne 1993). Lignin and suberin synthesis occur even in pathogen-infected elm tissue cultures ©2017 International Society of Arboriculture
November 2017
| Title Name |
Pages |
Delete |
Url |
| Empty |
Ai generated response may be inaccurate.
Search Text Block
Page #page_num
#doc_title
Hi $receivername|$receiveremail,
$sendername|$senderemail wrote these comments for you:
$message
$sendername|$senderemail would like for you to view the following digital edition.
Please click on the page below to be directed to the digital edition:
$thumbnail$pagenum
$link$pagenum
Your form submission was a success.
Downloading PDF
Generating your PDF, please wait...
This process might take longer please wait
