Lecture 10c: Plant-Pathogen Fungi and OOmycetes Flashcards

1
Q

Root rot diseases

A
  • if symptoms appear aboveground, the plants usually fail to recover
  • Browning and softening of root tips, root lesions that vary in size and colour, yellowing and wilting of leaves, stunted plant growth, reduced yield and crop loss
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2
Q

Host and pathogen white root rot

A

Host: Avacado
Pathogen: Rosellinia necatrix
detected in Limpopo, MPU, KZN

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3
Q

Info about Rosellinia necatrix

A
  • Soil-borne
  • Necrotroph
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4
Q

Symptoms of white root rot of avocados

A
  • above-ground: only noticeable once infection is established, Leaf yellowing, wilting and drying on the tree
    *White, cottony mycelial growth: roots, soill surface, Underneath the bark at the crown
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5
Q

Sexual spore disease cycle of white root rot of avocados

A
  • Easily found in infected tissue
  • Sexual structures formed inside perithecium
  • Perithecium takes a long time to develop and only under natural conditions
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6
Q

Asexual spore disease cyce of white root rot of avocados

A
  • Clamidospores: Rarely found under natural conditions
  • Conidiospores: Conidia originate at the ends of synnemata
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7
Q

penetrate avocado roots through

A
  • Natural openings
  • Wounds
  • Direct penetration via a sclerotium
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8
Q

Epidemiology of white root rot of avocados

A
  • Soil moister is the most important factor
  • Mycelium growth requires good aeration
  • mycelium growth is inhibited by light
  • Organic material enhances mycelium growth
  • Unlike most fungi, R.necatrix growth is not inhibited by high pH
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9
Q

Management of white root rot of avocados

A
  • Physical control: soil solarisation, soil/root barriers to prevent mycelium spread, burning infected trees
  • Resistant roots
  • Chem control: Chloropicrin, fungicides, fluazinam
  • Biological control: Mycoviruses, Trichoderma spp., Bacteria (Bacillus subtilis)
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10
Q

Phytophthora root of avocados H&P

A

Host: avocado
Pathogen: Phytophthora cinnamomi

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11
Q

Symptoms of phytophtora root rot of avocados

A
  • Chlorosis, brown necrotic leaf tips and space foliage
  • Infected feeder roots turn black and brittle
  • Redis-brown crankers (not common in avos)
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12
Q

Phytophthora cinnamomi characteristics

A
  • Soil-borne pathogen
  • Hemi-biotroph
  • Large host range
  • First discovered in SA
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13
Q

Zoospore disease cycle of phytophthora root rot of avocados

A

Swim through water filled soil pores, atrracted by root exudates

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13
Q

Chalmydospore disease cycle of phytophthora root rot of avocados

A

Are thick-walled spores (asexual). Survival structures

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13
Q

Sporangia disease cycle of Phytophthora root rot of avocados

A
  • Release zoospores in flooded or water-saturated soils
  • Form on the surface of infected tissue
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14
Q

Oospore disease cycle of Phytophthora root rot of avocados

A

Produced during sexual reproduction- Rare in nature. Also produce sporangia under favourable conditions

15
Q

Epidemiology of Phytophthora root rot of avocados

A
  • Disease cycle usually starts with infected feeder root or chlamydospores producing sporangia in warm, moist and aerated soils
  • High soil moisture
  • Low moisture and excess salt can also injure roots
16
Q

Management of Phytophthora root rot of avocados

A
  • Site selection
  • Avocado rootstock selection
  • Irrigation management
  • Chemical control: Potassium phosphate trunk injections
  • Organic amendments: Mulching practices
17
Q

Cankers

A
  • Under the bark of infected stems and branches
  • Expands and eventually kills branch
18
Q

Host and pathogen of black pod disease

A

Host: Cocoa
Pathogen: Phytophthora spp.

P.magakarya (West Africa)
P.capsici/citrophthora (Central and Southern America)
P.palmivora (Global)

19
Q

Symptoms of Black pod disease of cocoa

A

*On Pods: translucent spots on the pod surface that develop into dark spots. Entire pod becomes black. White and yellow downy growth on black areas. Infection spreads from outer husk to beans
* Cankers: Crankers at base of truck may extend to main roots. Canker lesions exude a reddish gum, do not penetrate deep into wood. Reduce tree vigour and pod carrying capacity, thus reducing yield

20
Q

Phytophthora palmivora

A
  • Pathogen attacks all parts of the cocoa tree at all stages of the growing cycle
  • Explosive epidemics during favourable environmental conditions
21
Q

Zoospore diease cycle of black pod disease

A
  • Discharged from sporangium
  • Motile
  • Proceed to entry point into the plant
22
Q

Oospore disease cycle of black pod disease

A
  • Rare in nature
  • A1 and A2 mating types required for sexual reproduction
23
Q

Sporangium

A
  • Produced on infected fruit, leaves, stems and roots
  • Germinate directly on the plant surface or in the soil
24
Q

Chlamydospore disease cycle of black pod disease

A
  • Mummified pods provide a reservoir of inoculum
  • Under humid conditions a single pod may produce up to 4 million sporangia
  • Disseminated by rain, ants, flying insects or rodents
25
Q

Epidemiology of black pod disease

A
  • Most severe during wet season
  • Zoospores encyst, germinate and penetrate host tissue within 30 mins when in free water on plant surface
  • P.palmivora survive ~10monthsin soil to tree canopy
  • Flying beetles carry inoculum into canopy and especially to pods
26
Q

Management of black pod disease

A
  • Quarantine
  • Resistance
  • Mulches and compost
  • Chemical control: Copper-based fungicides, potassium phosphate trunk injections
  • Cultural practices: Disposal of infected/ mummified pods. Ground covers decrease disease incidence