R2103 4.2 Describe the lifecycle of garden diseases

Question 1

Outline lifecycle of damping off

Rhizoctonia, Pythium fungi

Answer 1

1. Can survive in soil for many years as sclerotia
2. When soil reaches a certain temperature it secretes chemicals from its root as part of growth process
3. The secretions arouse the dormant sclerotia and the fungus produces a mass of long filaments (hyphae). They extend through the soil and make contact with the host plant
4. The mycelium release enzymes which dissolve cell walls
5. Then it colonises the intra and inter-cell spaces of the root tissue
6. Fungus then uses the plants nutrients
7. Once xylem tissue is killed the plant dies
8. The fungus produces survival structures - sclerotia - which are left in soil.

Question 2

Outline lifecycle of clubroot

Plasmodiophora brassicae - slimy fungus

Answer 2

3 main stages in life cycle:
1. Survival in soil - can survive up to 20 years as resting spores
2. Root hair infection - resting spores release mobile zoospores and when these reach surface of root hair, they penetrate through the cell wall and proliferate inside the root hairs
3. Cortical infection - more zoospores penetrate the cortical tissues of the main roots and gall are formed. This is by plasmodium stimulating cell division.
4. New generation of resting spores are formed and release back into soil as survival structures
5. Favours high temp, high soil moisture and acid soils
6. Club root can infect whenever the soil is moist and warm, so most new infections occur from mid-summer until late autumn.

Question 3

Outline lifecycle of potato blight

Phytophthora infestans - fungus

Answer 3

1. Spores overwinter in infected tubers - living tissue only
2. Develop and release airborne spores which land directly on potato / tomato foliage and either germinate directly or release swimming zoospores - so water is essential for the spread of the spores
3. Pathogen penetrates leaf or stem and grows into a black lesion that releases thousands on new spores. Carried by wind or rain splash - can infect plants miles away
4. Zoospores can reach tubers and cause them to rot. Infected seed tubers can then start a new life cycle of infection in following season
5. Sarpo Mira is a resistant cultivar
6. In the UK, outbreaks may occur from June onwards, usually earliest in the South West.

Question 4

Outline lifecycle of honey fungus

Armillaria mellea

Answer 4

1. Spreads underground. Either by direct contact between roots of healthy and infected plants also by means of black root-like structures called rhizomorphs ‘bootlaces’ which can spread from infected roots as deep as 18 inches at up to 1m a year.
2. Clumps of honey coloured toadstools not important in life cycle of fungus.
3. Generally caused by the spread of one individual example of the fungus - not the spreading spore stage. Can travel up to 30m
4. An infected tree stump can be a source of infection for 20 years or more
5. Often establishes itself from a tree or bush that has been planted too deeply

Question 5

Outline lifecycle of bacterial canker

Pseudomonas syringae - bacteria

Answer 5

1. Spring - bacteria transmitted in rain enters leaves through stomata causing infection visable as ‘shot-holes’; in later years cankers grow rapidly in spring. Also can be carried by insects
2. Summer - bacteria more or less dormant
3. Autumn - bateria enter leaves through wounds and leaf scars; in later years bacteria remain around edges of canker to be transmitted in spring rain

Question 6

Outline lifecycle of potato leaf curl virus

Answer 6

1. Winged aphids spread the virus; winged and wingless feed on infected plants; in autumn males are produced, eggs laid on winter host - not infected with virus
2. Virus overwinters in stored tubers and in weed on Solanaceae
3. Infected tubers grow: In spring peach-potato aphids feed on these and aquire lifetime infection with virus; winged generation feed, aquire virus and move on to new plant