diseases caused by fungi

Question 1

saprotrophs

Answer 1

fungi that colonise and degrade dead material

Question 2

necrotrophs

Answer 2

fungi that colonise living (often weakened) hosts and kills them

Question 3

biotrophs

Answer 3

fungi that colonise living hosts but do not cause host death

Question 4

fungal diseases in warm blooded animals

Answer 4

• few fungi can grow well at 37C so there are not many fungal diseases of warm blooded animals
• very difficult to treat due to similar cell structure and metabolic pathways
• diseases mostly on extremities where temperatures are slightly lower (35-36C) and the immune response is less efficient
• mammal as and birds possibly raised body temperature during evolution in part to avoid fungal attack

Question 5

aspergillosis

Answer 5

• human lung disease caused by Aspergillus fumigatus fungus
• small spores 2-3 micrometers inhaled deep into lungs
• often fatal for immunocompromised people, difficult to get rid of
• similar disease caused by Penicillium marneffi in East Asia

Question 6

Candidiasis

Answer 6

• human disease of mucosal membranes caused by Candida albicans
• yeast-mycelial dimorphism
• immunocompromised people, babies

Question 7

insect diseases, Metarhizium acridum

Answer 7

• locust-specific pathogen
• spreads during locust swarms
• optimum temperature is 28C, growth stops at 42C
• infected locusts sunbathe to raise temps up to 44C to inhibit pathogen spread, ‘behavioural fever’
• possible uses as a biocontrol agent

Question 8

bats, white-nose syndrome

Answer 8

• in USA
• Geomyces destructans
• psychropile (likes cold) so infects during hibernation

Question 9

inhalation of fungal allergens

Answer 9

• overly defensive immune systems can react to surface antigens on fungal spores
• e.g. from pillows, moist and made from cellulose so ideal conditions for fungal growth

Question 10

plant diseases, dutch elm disease

Answer 10

• Ophiosotma ulmi (ascomycete)
• necrotrophic pathogen, weak saprotroph, relies on beetle vector to transport it to living trees in spring
• vascular wilt, blocks xylem, blocking water supply to leaves, leaves wilt and drop
• cerato-ulmi hydrophobins coat vascular tissue in tree
• unilateral infections, one side of tree vascular bundles infected, other side usually infected the next year via vector
• 1920s and 1970s (diseased logs from US) epidemics
• bark beetle vector (Scolytus), flies at ~5m, some short elms have survived

Question 11

disease control against dutch elm disease

Answer 11

• none effective
• breeding resistant strains
• sanitation, burning infected trees
• fungicides, impractical for large trees

Question 12

dutch elm disease life cycle

Answer 12

• entry into sap via beetle feeding wound
• spreads within host via y-form and spreads through vascular systems (yeast-mycelial dimorphisms)
• xylem blockage/vascular wilt symptoms lead to host death
• saprotrophic in bark on dead host, growth in beetle breeding galleries
• spores picked up by beetle

Question 13

late blight

Answer 13

• P.infestans
• infects potatoes and tomatoes, decreased yield and rot when stored
• oomycetes, FLO
• evolved in Central America
• arrived US 1843, Europe 1845
• leaves develop lesions and become necrotic, defoliage
• 75% defoliage means end of tuber growth
• splashes of rainfall spreads oomycete to new leaves

Question 14

UK epidemiology of late blight UK

Answer 14

• needs cool wet weather
• UK potato growing industry is in East England (warm and dry, relies on irrigation)
• Smith/Beaumont period = 2 days of low temperature and high humidity, causes a blight outbreak
• disease lesions show 2-3 weeks later
• crops need to be sprayed with pesticide immediately after period
• tend to be June-August

Question 15

dispersal of late blight

Answer 15

• night time sporulation from sporangia on the surface of leaves detach and blow away (zoospores have to be encysted before morning when the dew drop evaporates)
• sporangia settle on dewdrops on leaves, zoospores release, swim down and encyst onto leaf (stick to leaf and lose flagella)
• germ tube and penetration peg formed

Question 16

late blight infection process

Answer 16

1. zoospore encysts, penetration peg forms (narrow hypha coated with enzymes, large osmotic pressure, pushes through cell wall, invaginates membrane), 12hrs
2. latent phase, nutrient uptake, intracellular spread with crude haustoria, 3-4 days
3. necrotrophy, pectinases causes plant tissues to fall apart (pectin holds cells together), host death, escape via stomata, sporulating structures on leaf surface, 6-7 days
4. overwintering, oospores sit in soil until next crop

Question 17

late blight, sexual reproduction

Answer 17

• has A1 and A2 mating types, both have male and female parts but an A1 and A2 type is needed for sexual reproduction
• only A1 introduced into Europe and US from 1845-1970s
• no sexual reproduction/oomycete formation, survived in leftover potatoes over winter
• A2 arrived in Europe in 1970s from Mexico (poor UK crop, potatoes imported), oospores observed

Question 18

disease control of late blight

Answer 18

• can slow disease progress
• breeding resistance, often using detached leaf assay, e.g. Maris Piper, have to also take into account yield and quality when breeding crops
• sanitation, crop rotation, removing inoculum using herbicide to remove diseased tubers
• pesticides, resistance problems, copper vs phenylamide pesticides
• Blitecast = computer predictions of blight, timing pesticide application

Question 19

Bordeaux mixture

Answer 19

• 1st pesticide
• produced in France in 1870s against downy mildew on grapes
• CuSO4 and lime
• lime makes CuSO4 less soluble, not washed away with rain
• effective against oomycetes but not true fungi

Question 20

biotrophic pathogens

Answer 20

• rusts, smuts and powdery mildews
• form appressoria to infect and feeding haustoria
• appressoria have high osmotic pressure to enter host
• haustoria bypass cell wall for a more efficient transfer of nutrients
• highly evolved, do not kill host

Question 21

stem rust

Answer 21

• lots of different spore types
• heteroecious, on cereal crops, barberry is overwintering host
• removal of barberry in UK 17-19th century inhibited sexual cycle (more rust in fields near barberry trees observed)
• strain Ug99 found in Uganda in 1999, spreading across Asia and Africa causing crop loss

Question 22

coffee rust

Answer 22

• Hemileia vastatrix
• discovered Kenya 1861, spread to Asia
• Sri Lankan coffee exports dropped >95% 1869-1890, switched to tea growing

Question 23

powdery mildews

Answer 23

• ascomycetes
• obligate biotrophs
• overwintering cleistothecia (closed cup containing ascospores)
• also make spider like haustoria
• Blumeria graminis var hordei is barley variety
• screening for resistance using detached leaf assay (spores observed)

Question 24

Blumeria (powdery mildews) spores, host sensing

Answer 24

1. spore lands on surface
2. grows primary germ tube that chemically senses surface
3. produces secondary germ tube and appressorium/infection peg if it has landed on host

Question 25

plant resistance

Answer 25

• host must detect fungus quickly
• fungal attack is stopped by programmed cell death
  e.g. Rice blast disease, resistant strain creates H2O2, harnessing the Fenton reaction to kill the infected cell