unit two

chryphnoctria parasitica

causual agent of chestnut blight

what does chestnut blight have in common with dutch elm disease?

• both pathogens are exotic; came out of asia

• they decimated iconic tree species (american chestnut, american elm)

• both are ascomycetes

how does cyrophenctria parasitica reproduce sexually and asexually?

• produce asexual conidia in a pycnidium

• produce sexual ascospores in a perithecium

pycnidium

flask-shaped asexual fruiting body

perithecium

flash-shaped sexual fruiting body containing asci and ascospores

what sort of symptoms/signs does chestnut blight show?

large cankers at the base of the stem

• these cankers can be a meter in length

• girdling stem at the base, which means if the tree dies, the roots stay alive and produce new shoots

extent of tree death caused by chestnut blight

killed 4 billion american chestnuts

• lowered the timber value in the eastern us by 15%

• they were replaced by oaks, which are now dominant in hardwood forests because of the disappearance of chestnuts

why was there so much tree death in chestnuts?

the american chestnut had never encountered this pathogen before, so it did not have the ability to fight it off or develop resistance against the pathogen, which is what made it so devastating

• similar to COVID-19 hitting human population

when was cryphonectria parasitica introduce to the US?

1904

• an exotic chestnut was introduced into the bronx botanical garden, which introduced the pathogen

• killed 4 billion chestnuts

• no host resistance in the american chestnut

what can be done about the devastation of chestnut blight?

hybridization, biological control, or transgenic control

are all control efforts 100% successful?

no, these are mostly attempts to control

hybridization

• attempt with asian chestnuts, which are resistant

• species that are native to asia have host resistance, so hybridization could make american chestnuts more resistant

• there still needs to be more research

biological control

• looks promising in europe, but not so much the US

• control with mycovirus

mycovirus

viruses that can affect fungi;bacteria

• attacks fungi and causes hypovirulence

• when you infect a pathogen with a virus, they become sick

• disease is being reduce because of the reduction in growth

• however, viruses are hard to transmit

how are mycoviruses administered?

inoculate the strain with the virus into the canker and it makes the pathogen hypovirulent

hypovirulence

lowering relative ability to cause disease

transgenic chestnuts

• genetically modified chestnuts with the oxalate oxidase (enzyme) gene from wheat

• it moves the gene from one species to another (wheat to chestnut)

• it breaks down the oxalate so the host is able to suppress the pathogenicity factor

• this permits resistance to the disease

chestnut blight

• pathogenic fungus Cyrophonectria parasitica

• member of the ascomycota (sac fung) it’s an ascomycete

• necrotrophic fungus native to east asia and south east asia

• introduced to europe and north american in early 1900s

• spread rapidly and caused tree loss

how does the pathogen in chestnut blight persist?

windborne ascospores and conidia distributed by rainsplash

what are the signs/symptoms of chestnut blight?

• fungus enters through wounds on susceptible trees and grows under the bark, killing the cambium all the way around twig, branch or trunk

• first symptom is small orange-brown area on bark

• sunken canker forms as the mycelial fan spreads under the bark

• they produce toxic compounds (oxalic acid) and it reduces the pH of infected issue, which is toxic to plant cells

• canker girgles the tree, killing everything above it

• distinct yellow conidia tendrils can be seen in wet weather

what type of parasite is cryphonectria parasitica?

obligate parasite

obligate parasite

only survive on living host

dutch elm disease

ophiostoma ulmi or ophiostoma novo-ulmi (north america and europe)

where was dutch elm disease first identified?

netherlands in 1921

what elm species is particularly susceptible to dutch elm disease?

Ulmus americana

what are the symptoms and signs of dutch elm disease?

• vascular wilt disease

• earliest external symptoms of infection are the yellowing and wilting (flagging) of leaves on individual branches

• these leaves turn brown and curl up as the branches die, and the leaves may drop

• initially only a party of the tree crown may be affected

• symptoms may progress rapidly throughout the crown

• highly susceptible trees usually die in a year, but some linger for years

when do symptoms progress the quickest in dutch elm disease? why?

• symptoms progress quickly and death happens quickly in trees infected in early spring

• trees infected later in the summer may survive longer

explain symptoms and signs of the bark caused by dutch elm disease

• if the bark of infected elm twigs or branches is peeled back, brown discoloration is seed on outer layer of wood

• discoloration in xylem occurs before foliar symptoms

• xylem browning is discontinuous

what do you see in the xylem browning of dutch elm disease?

• discontinuous

• cross section shows circle of brown dots or a ring

why do you need labratory culturing and identification of the dutch elm fungus?

• other wilt diseases of elm (Verticillium wilt) also cause sapwood discoloration so you need lab culturing and ID of fungus to identify which disease it is

why do foliar symptoms result?

sap flow ceases in the infected wood

where are the signs of dutch elm disease?

fungal structures of the pathogens are found within infected elm trees

explain the pathogen biology of dutch elm disease

• Ophiostoma species that cause Dutch elm disease grow and reproduce only within elms

• they’re facultative saprophytes

• Ophiostoma ulmi caused the original Dutch elm disease epidemic in europe and north america in the mid 1900s

• Ophiostoma novo-ulmi, even more aggressive, replaced former pathogen

ophiostoma sp. pathogen

fungi spread within stems and roots of living elms both by passive transport of spores and by mycelial growth of colonies initiated by spores that germinate in the xylem

• mycelium is creamy whit

• composed of septate hyphae with haploid nuclei

where do the spores of ophiostoma germinate?

the xylem

what color is the mycelium of ophiostoma?

• creamy white, septate hyphae with haploid nuclei

asexual reproduction of ophiostoma ulmi and O. novo-ulmi

• two asexual forms that produce asexual spores called conidia

• in the xylem vessels of living elm trees, small, white, oval conidia are formed in clusers on short mycelial brances

• the conidia are carried in xylem vessels where they reproduce by budding, germinate to produce mycelium, and thus spread the disease throughout the tree

how are conidia in ophiostoma sp. produced?

in dying or recently dead trees, conidia are produced by mycelium growing in the bark and in tunnels created by beetles just under the bark

• they’re produced at the tip of 1-2mm tall synnemata

explain the synnema of ophiostoma sp.

each synnema consists of hyphae fused to form an erect, dark stalk with a round, nearly colorless head of sticky spores

• beetle vectors cary the stick spores to new elm trees

synnema

stalk (dutch elm disease)

• sticky conidia on the stalk, beetles carry it around

explain the sexual reproduction of ophiostoma sp.

• based on sexual stage structure, this pathogen is an ascomycete genus Ophiostoma

• when two mating types come in contact, ascospores are produced in sperical, black, long-necked perithecia

  • perithecia form in the bark, either singly or in groups.

  • ascospores are produced in the asci that degenerate inside of the perithecia

  • free ascospores are discharged at the opening of the perithecial neck where they accumulate in sticky droplets that may be disseminated by beetle vectors

disease cycle and epidemiology of dutch elm disease

pathogens overwinter in the bark and outer wood of dying or recently dead elm trees and in elm logs as mycelia and synnemata with conidia

• fungi are spread from their sites by their vectors (elm bark beetles)

• two beetle species spread the pathogens in North America: Scolytus multistriatus and Hylurgopinus rufipes

what are the two beetles species that can spread the pathogens around in north America? how do they act as vectors for ophiostoma sp.?

• smaller European elm bark beetle: Scolytus multistriatus

• native elm bark beetle: Hylurgopinus rufipes

  • adult female bores through the bark of dead or dying elm trees and elm logs and creates a tunnel in the wood as she feeds

  • she lays eggs in the tunnel behind her

  • the eggs hatch into larvae that being to feed, creating tunnels at right angles to the maternal tunnel

what is the resulting pattern of tunnels created by elm bark beetles called?

it’s called a gallery

• larvae pupate and emerge through the bark as adultsif fungi are present in the tree or log, emerging adults carry thousands of sticky conidia on their bodies

how do newly emerged bark beetles feed?

• S. multristriatus adults feed in the twig crotches of elm branches

  • H. rufipes adults tunnel in the bark of the elm branches and trunks

how are fungal spores spread by elm bark beetles?

• as beetles feed, fungal spores are deposited

• beetle vectors only feed on healthy elms for a few days

  • then they fly to dying or recently dead elm trees or to freshly cut elm wood to feed, create galleries, and lay eggs

  • the spores dislodged from the elm bark beetles in feeding wounds and tunnels germinate and produce mycelium that grows into the xylem the mycelium produces millions of small, white, oval conidia that spread through the xylem sap (vascular disease)

does the fungi produce toxins (ophiostoma)?

• yes, they produce enzymes and toxins that degrade plant cell walls and kill xylem parenchyma cells

• they also induce hormonal imbalance that leads to the development of tyloses overgrowths of parenchyma cells that push into and block the water-conduction xylem cells

• blockage of the xylem by tyloses and gums (thought to be products of plant cell wall breakdown) causes wilting leaves

• killing of xylem parenchyma cells also causes brown discoloration just under the bark

epidemiology of the dutch elm disease

infections that take place in spring or early summer involve springwood which has very long xylem vessels

• this makes it easier for fungi to spread rapidly throughout the tree, which will then die very quickly

• later in the season, the fungi are restricted to shorter vessels of summerwood

  • fungi spread slowly in the tree

what kind of infections result in the elm tree surviving for longer? (Ophiostoma sp.)

localized infections result because of summerwood and later summer season infections, and the tree survives for longer

how do healthy elm treeds being infected by the dutch elm disease?

• they can become infected by the feeding spore-contaminated elm bark beetles or through the development of grafts between their roots and the roots of infected trees

• trees infected by beetle vectors develop dymptoms first in an upper section of the crown

• trees graft infected via root grafts first develop symptoms in the lower crown

how does infection by root grafts in elm trees function?

when fungi are introduced through root graft, they distribute quicker throughout the tree in the vascular system and the entire tree can wilt and die

• root grafts form naturally bewtween closely spaced elm trees with intertwined roots

• large elms growing within 20 feet of each other have a 100% chance of root graft infection

• likelihood of spread is lower when they’re at least 40 feet apart

what does the severity and rate of spread of dutch elm disease depend on?

• species of pathogen

• how rapidly elm bark beetles reproduce

• level of susceptibility of elm hosts

• environment

• temperature around 68 F favor formation of conidia

• perithecia are induced at 46 to 50 F

what is the ideal temperature for ophiostoma sp.

68F favors conidia formation

46-50F favors perithecia induction

what are the three similarities between dutch elm disease and chestnut blight?

1. both exotic pathogens (Asia)

2. both decimated popular tree species (american chestnut and american elm)

3. both ascomycetes

what happens to elm population impacted by ophiostoma sp.

without effective management, dutch elm disease increases exponentially until an affected elm population is greatly depleted

• seedlings and saplings can escape and live long enough to reproduce, so even the most susceptible elm species have never been threatened by extinction

• wild elm populations have increased in easter and midwestern US, leading to renewed prominence of dutch elm disease in landscapes

what are the three different types of dutch elm management?

• cultural strategies

• chemical strategies

• breeding for resistance (transgenics)

what are the cultural strategies for managing dutch elm disease?

• it’s cheapter to manage the disease than to remove the large dead trees that it leaves behind

• some communities focus on cultural practices for disease management, including avoiding monocultures of elm trees, removal of all dying or recently dead branches, trees, and cut wood (sanitaton) and the breakage of root grafts between adjacent elms

• success depends on diligent insepction of all elm trees in area several times each growing season

• wood must be burned, chipped, or buried so it can’t provide a home for beetle vectors

what is organized community sanitation for dutch elm disease?

• can delay the loss of elms

• the time when half of the elm trees in an area have been lost can be delayed between 7 to 30 years

• if privately owned trees are included in inspection and mandatory removal, the longer end of this range is likely

• this is mostly a delaying tactic (burn wood)

what are chemical strategies for managing dutch elm disease?

• in the past, insecticides were sprayed on elm trees in attempts to kill the beetle vectors of dutch elm disease

• management strategy was expensive and not effective, and people attacked it because of concern about wildlife and people

• fungicides are relatively expensive, non are completely effective

• chemical management is used only to protect elm trees of high value (DC Mall, well-maintained properties)

breeding for resistance to fight ophiostoma sp.

• long term solution

• development of disease-resistant cultivars of elms

• several asian elm species have moderate to high resistance

• breeding programs introduced resistance from asian elm species to native elm speces

• american elm breeders want to maintain the shape of the american elm, which is what makes it a desirable shade tree

• several hybrid and clonal elms are available that have good resistance to dutch elm disease

historical signification of ophiostoma sp.

• first elm disease epidemic started when it was introduced in 1920s by furniture makers

  • used imported European elm logs to make veneer for cabinets and tables

  • some of the beetle vectors were brought from Europe years before the fungi were introduced

• when more aggressive O. novo-ulmi was later introduced to America, it killed many elms that survived the original epidemic

• the spread of this disease highlights the danger of moving plant materials around the world

who first identified the causal agent of the dutch elm disease?

• dutch scientist, Marie Beatrice Schwarz

who first identified dutch elm disease in ohio in 1930?

• dutch scientist, Chrstine Johanna Buisman (she had seen the disease in her homeland) first identified it in ohio in 1930

how did ophoistoma geographically spread?

spread up and down the east cost and west across the contient

• reached west coast in 1973

• over 40 million american elm trees have been killed by this disease, and today it is still a very destructive disease of shade tree

impact of elm monocultures

• they were planted in rows along streets and walkways

• provided effective windbreaks

• shady canopies

• dense plantings are monocultures

monocultures

when plants of the same species are grown in close proximity with few other types of plants prsent

• people have planted these for hundreds of years

• provide uniformity

  • aesthetic and production practices

  • planting, management, and harvest are simpler when one kind of plant is grown in an area

dangers of monocultures

they’re subject to the same catastrophic problems because they’re the same species

• disease, insect, or weather condition that harms one plant harms the rest too

• this is what made dutch elm disease so devastating in towns and cities

• pathogens can move closely between closely spaced trees through insect vectors or root grafts, leaving destruction

• this highlights why diversity is important

explain transgenics in american chestnuts

• American chestnut is engineered to express wheat oxilate oxidase. This enzyme degrades the oxalate produced by the pathogen, resistance of Cryphonectria parasitica

  • oxalate oxidase enzyme

  • this enzyme converts oxalic acid into hydrogen peroxide and carbon dioxide

  • when the fungus attempts to penetrate the chestnut, the oxalic acid produced by the fungus is broken down by the enzyme, creating hydrogen peroxide

  • hydrogen peroxide is toxic to the fungus

  • this kills the fungus

what are the ascomycetes we discussed in class?

powdery mildews, apple scab, dutch elm disease, chestnut blight, block spot of rose

deuteromycetes

aka anamorphic

fungi with no known sexual stage

they only have asexual reproduction

deuteromycetes and ascomycetes

includes many species causing leaf/fruit spots, blights, cankers, root rots, vascular wilts, anthracnose, also powdery mildews

deuteromycetes are imperfect, ascomycetes are perfect fungi

both eukaryotic species

ascomycetes

• both sexual and asexual reproduction

• sac fungi

• largest phylum of the kingdom fungi

• ascus - sexual structure that produces ascospores, some produce asexual only

deuteromycetes are considered the anamorphic stages of ascomycetes, they produce spores asexually through sporogenesis

one fungus one name

many fungi, especially ascomycetes used to have 2 scientific names (sexual and asexual) (telemorph and anamorph respectively)

since 2013, only ONE name (holomorph) is applied

cell wall composition of mitosporic fungi

made of chitin and not cellulose

examples of mitosporic fungi

• Alternaria

• Botrytis

• Colletotrichum

• Fusarium

• Pythium

asexual spore types

• always conidia

conidia

nonmotile, germinate directly

four asexual fruiting bodies

pycnidium, acervulus, sporodocnium, synnema

pycnidium

• flask shaped

• chestnut blight

acervulus

subepidermal saucer, anthracnose

sporodochium

• cushion

• peach brown rot

synnema

• stalk

• dutch elm disease

chlamydospores, sclerotia

both are survival structures

chlamydospores

thick-walled resting structures produced by some fungi that allow them to survive unfavorable conditions

thickening and hardening of fungal cell wall, provides protection

asexual spores

source of inoculum

can remain dormant for extended period untl conditions are right to germinate

sclerotia

compact, hardened structures that are formed by fungi for survival

hyphal tissue that has become compacted and encased in a thick, protective layer