The importance of mycorrhizae and the role they play in enhancing tree health is widely recognized by arborists. However, the role of endophytes, i.e., bacteria and fungi that live within a tree without causing any disease symptoms, is greatly unappreciated. The aim of this article is to provide a brief overview highlighting the benefits of endophytes, especially their potential as bio-control agents.
What are endophytes?
Endophytes are bacteria and fungi that live within healthy tree tissue (sapwood, leaves, roots, stem, bark, buds) without causing any visible symptoms of disease. Of all the world’s plants, only a few grass species have had their complete complement of endophytes studied. In contrast, endophytes associated with trees are poorly known.
Commonly isolated endophytes from trees include Enterobacter spp., Colletotrichum spp., Phomopsis spp., Phyllosticta spp., Cladosporium spp. and Streptomycetes. However, endophytic populations vary between tree species. Within oaks (Quercus), species such as Penicillum, Trichoderma, Botryosphaeria, Cladosporium, Coryneum, Cytospora, Discula and Stereum tend to dominate. Whereas within ash (Fraxinus), Aureobasidium, Phoma, Dothideomycetes, Diaporthe and Lophiostoma species are more prevalent.
Role of endophytes – growth
Endophytes play an important role in helping trees uptake essential nutrients from the soil. They help tree roots absorb phosphorous in phosphorous-deficient soils and can aid in the uptake of nitrogen in soils of low fertility, as well as allow trees to establish on sites contaminated with heavy metals. In addition, endophytes produce a range of hormones, such as auxins, cytokinins and gibberellic acids, that have essential roles in tree growth, seed and flower set, senescence (biological aging) and dormancy.
Burkholderia vietnamiensis, for example, is an endophytic bacterium isolated from poplar trees that produces the hormone auxin, responsible for root formation. This, in turn, has been shown to stimulate root growth following construction damage, helping a poplar tree survive. Endophytes also produce or stimulate the production of chemicals in leaves that protect trees from drought, heavy-metal toxicity, heat waves and high light intensity, problems trees routinely encounter in urban landscapes.
Disease resistance
Endophytes are now regarded as an untapped source of biological-control agents that offer an alternative to chemical control. They act in one of two ways:
- As a direct biological-control agent against a specific disease or pest.
- By the production of chemical compounds that inhibit fungal and bacterial growth.
Endophytes as direct bio-control agents
It has been shown that an endophytic fungus living in the inner bark of elm trees reduced the movement of a bark beetle spreading Dutch elm disease. Another example is the endophytic fungi Deadelea quercina, which invades the galls of a cynipid insect and kills the wasp inside them. The endophytic fungi Beauveria bassiana has been found to control borer insects in coffee, and is now available as a commercial product (Naturalis), widely used for the management of thrips, mites, white fly and aphids (egg, larvae, adult).
In Western white pine (Pinus monticola), fungal endophytes reduced damage by white pine blister rust (Cronartium ribicola). Seedlings colonized with fungal endophytes had greater survival rates than control seedlings, with the levels of resistance observed the same as white pine trees selected in breeding programs for resistance against this disease. When a mixture of six different endophytes isolated from cacao (Theobroma cacao L.) trees was used to inoculate leaves of endophyte-free seedlings of the same plant species, the severity of Phytophthora infection on leaves was significantly reduced.
The endophyte Trichoderma citrinoviride, isolated from cork oak, was shown to possess a strong biological activity against the main pathogens causing oak decline within Mediterranean climates. Recent work at the Research Institute for Landscape and Ornamental Gardens in the Czech Republic showed that, in the endophytic fungal community of trees resilient to ash dieback, Trichoderma citrinoviride significantly reduced ash-dieback (Hymenoscyphus fraxineus) growth by anywhere from 42% to 83% under laboratory conditions.
Chemicals produced by endophytes
Many endophytes produce substances that can inhibit the growth of several disease-causing fungi. When produced by endophytes in or on the tree, these substances can constitute a powerful defense mechanism. Compounds produced by endophytic microbes include Taxol (anticancer drug), Cryptocin (antifungal agent), Jesterone (antifungal agent), Oocydin (antifungal agent) and Ambuic acid (antifungal agent).
Of particular interest is the cancer-fighting drug Taxol, which has shown to be produced by 18 different fungi, primarily endophytes that live within Taxus (yew) and other trees. As Taxol is a fungicide, and the endophyte(s) producing it is (are) resistant to Taxol, this endophyte strategy has been used to prevent fungal attack by pathogens such as Perenniporia subacida (poroid fungus of Douglas fir), Phaeolus schweinitzii (velvet-top fungus of fir, spruce and hemlock, pine and larch) and Heterobasidion annosum (a decay fungi of conifer trees) at minimal “metabolic” cost to the tree.
Liquid cultures of Trichoderma citrinoviride, derived from cork oak, produced a mixture of antibiotics that demonstrated antifungal activity against seven forest-tree pathogens to include Apiognomonia quercina (oak anthracnose), Botryosphaeria corticola, B. parva, B. obtuse (canker diseases of oak), Biscognauxia mediterranea (charcoal canker of hardwood trees) and Diplodia pinea and D. scrobiculata (tip blights of pine).
Importance of endophytes in the U.S.
In 2018, more than 6 million acres of tree mortality was caused by insect pests and diseases in the United States. In forests across the western U.S., for example, tree deaths by the bark beetle fir engraver accounted for 33% of total mortality, resulting in 1.9 million acres of dead white, red and grand fir. Spruce beetle, emerald ash borer and sudden oak death were also important sources of tree mortality throughout U.S. forests in 2018.
In addition to tree death, defoliating pests damage trees by eating leaves or needles, causing significant losses of foliage that in turn detrimentally impact forest health. While a “one-off” defoliation usually does not result in tree death, repeated attacks in combination with heat waves and drought, for example, can cause trees that would normally recover to succumb to these defoliating insects.
Case scenerio 1
Oak wilt (Ceratocystis fagacearum) is a highly destructive fungus that causes wilt in oak trees in the Eastern and Midwestern U.S. (Photo 1) Extensive damage occurs particularly to red-oak species, while damage to white oaks tends to be less severe. Presently, there is no cure for oak-wilt-infected trees. Management relies heavily on avoiding or reducing infection in areas where the disease occurs and preventing its introduction to areas that are disease free.
Research from Texas A&M University isolated endophytic bacteria from several surviving live oaks (Quercus fusiformis) where oak wilt is epidemic, and evaluated their potential as biological control agents for oak wilt. Under laboratory conditions, 183 of the bacterial endophytes isolated showed inhibition of oak wilt. The six most powerful endophytes were then further evaluated using containerized Spanish (Quercus texana) and live oaks.
Trees that were pre-inoculated with the bacterial endophyte Pseudomonas denitrificans (stem injection) were then inoculated with the oak-wilt pathogen. These tests showed a 50% reduction in the number of diseased trees and a 17% decrease in crown loss. In a second separate trial, pre-inoculation with either P. denitrificans or P. putida also significantly reduced crown loss caused by oak wilt.
These studies show that there is a potential for the use of endophytic bacteria to control oak wilt.
Case scenerio 2
In a similar manner to the work at Texas A&M University, and as part of ongoing Bartlett Tree Expert-funded Ph.D. research at the University of Reading in the United Kingdom, the role of endophytes in conferring resistance to acute oak decline (AOD) was studied. (Photo 2) The trial site was a mature English-oak woodland in Writtle Forest, in Essex, where AOD is resulting in the death and decline of many oaks.
Endophytic fungi and bacteria from asymptomatic (showing no symptoms of AOD) and symptomatic (showing symptoms of AOD, e.g., bleeding trunk lesions) trees were isolated from trees located within the same area of woodland. This was achieved by removing cores of wood from both tree types, then growing fungi and bacteria from these cores on a number of different agar growth mediums. DNA was then extracted from any fungi that grew, followed by DNA sequencing.
Within asymptomatic oak trees, several species of Penicillium fungi were identified, including Penicillium aeneum, P.citreosulfuratum, P.miczyskii, P.brevicompactum, P.aurantiacobrunneum and P.manginii, as well as a number of other unidentified Penicillium species. (Photo 3) The benefits of penicillin as an antibiotic to treat bacterial diseases of humans, such as scarlet fever, pneumonia, meningitis and diphtheria, are now a part of history since their “accidental” discovery by Sir Alexander Fleming in 1928. Indeed, one of the endophytic Penicillium species identified inhibited the growth of Brenneria goodwinii and Gibbsiella quercinecans, two of the bacteria responsible for AOD.
Further work
In further work, one Bacillus and three Pseudomonas bacterial strains were shown to inhibit growth of Armillaria, a root disease, by up to 40-50%. Combining the two bacteria resulted in 65% inhibition. Such a result proves that oak trees showing no symptoms of AOD contain within their tissue fungal and bacterial endophytes that confer resistance against both AOD and Armillaria.
Future research
While resistance to tree diseases is generally considered a genetic trait, there is now a growing body of evidence showing that the influence of endophytes within a tree may have been underestimated. (Photo 4) Ongoing research at the Bartlett Tree Research Laboratory aims to elucidate whether a specific endophytic profile exists between susceptible and resilient oaks to AOD. If such a profile does exist, then this can be used to identify resilient and susceptible oaks.
There also would be the potential to apply endophytes with known bio-control properties into oaks that are lacking these endophytes to enhance resistance. Developments in molecular diagnostic technology now make this type of research a very real possibility that could prove of importance in developing new strategies to protect trees. Importantly, no genetic manipulation or chemicals are being used in this management strategy.
Glynn C. Percival, Ph.D., is the senior arboricultural researcher at the Bartlett Tree Research Laboratory based in Charlotte, North Carolina, a division of Bartlett Tree Experts, a 49-year TCIA member company.
