Lecture 10c: Plant-Pathogen Fungi and OOmycetes

Question 1

Root rot diseases

Answer 1

• 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

Question 2

Host and pathogen white root rot

Answer 2

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

Question 3

Info about Rosellinia necatrix

Answer 3

• Soil-borne
• Necrotroph

Question 4

Symptoms of white root rot of avocados

Answer 4

• 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

Question 5

Sexual spore disease cycle of white root rot of avocados

Answer 5

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

Question 6

Asexual spore disease cyce of white root rot of avocados

Answer 6

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

Question 7

penetrate avocado roots through

Answer 7

• Natural openings
• Wounds
• Direct penetration via a sclerotium

Question 8

Epidemiology of white root rot of avocados

Answer 8

• 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

Question 9

Management of white root rot of avocados

Answer 9

• 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)

Question 10

Phytophthora root of avocados H&P

Answer 10

Host: avocado
Pathogen: Phytophthora cinnamomi

Question 11

Symptoms of phytophtora root rot of avocados

Answer 11

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

Question 12

Phytophthora cinnamomi characteristics

Answer 12

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

Question 13

Zoospore disease cycle of phytophthora root rot of avocados

Answer 13

Swim through water filled soil pores, atrracted by root exudates

Question 13

Chalmydospore disease cycle of phytophthora root rot of avocados

Answer 13

Are thick-walled spores (asexual). Survival structures

Question 13

Sporangia disease cycle of Phytophthora root rot of avocados

Answer 13

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

Question 14

Oospore disease cycle of Phytophthora root rot of avocados

Answer 14

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

Question 15

Epidemiology of Phytophthora root rot of avocados

Answer 15

• 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

Question 16

Management of Phytophthora root rot of avocados

Answer 16

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

Question 17

Cankers

Answer 17

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

Question 18

Host and pathogen of black pod disease

Answer 18

Host: Cocoa
Pathogen: Phytophthora spp.

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

Question 19

Symptoms of Black pod disease of cocoa

Answer 19

*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

Question 20

Phytophthora palmivora

Answer 20

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

Question 21

Zoospore diease cycle of black pod disease

Answer 21

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

Question 22

Oospore disease cycle of black pod disease

Answer 22

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

Question 23

Sporangium

Answer 23

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

Question 24

Chlamydospore disease cycle of black pod disease

Answer 24

• 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

Question 25

Epidemiology of black pod disease

Answer 25

• 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

Question 26

Management of black pod disease

Answer 26

• 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