Lecture 18: post harvest pathology Flashcards
define disease
manifestation of the attacking pathogen resulting in the malfunctioning of host ells and their tissues
results from the continued attack and development of pathogens
define pathogen
microorganism that can incite a disease on the host
define a parasite
organism living on or in another living system (host) an causes the disease by obtaining its food from the latter
define an infection
establishment of a parasite (pathogen) within a host plant
define pathogenicity
capability of a pathogen to cause the disease
define symptom
external or internal reaction or alterations of a plant as a result of a disease
define sign
pathogen or parts or products seen on a host plant
define rats
softening, discoloration and disinegration of a succulent plant tissue as a result of fungal or bacterial infection
describe chlorosis
yellowing of normally green tissue due to chlorophyll destruction or failure of chlorophyll formation
what are lesions
visible disease area
what biotic or infectious agents causes plant diseases
fungi and bacteria
describe how a spore can cause disease
- disperses fungi
what abiotic or non-infectious factors causes plant disease?
- too low or high temp
- lack or excess H2O and light
- lack of O2
- air pollution
etc. ..
___ is required for spore germination
Oxygen
describe the process of spore germination
- absorption of moisture
- spore swelling due to: incr O2 consumption; rapid CO2 evolution; rapid incr in DNA, RNA and protein synthesis
- germ tube protrudes through spore coat
- germ tube lengthens and side branches are initiated
what are 2 modes of entry of pathogens into fruit?
- penetrating healthy tissue
2. enter through wounds and bruises
______ resists pathogen invasion
plant organs and protective tissues
how can you maintain a strong defence mechanism against pathogens?
- growing healthy tissues
for a pathogen to be successful, it must… (2)
- gain access to plant tissue
2. overcome defense response
what are the steps of direct penetration of fungi?
- landing of spore upon fruit
- germination
- adhesion of germ tube and appressoria (binding)
- branching and re-branching of the fruit flesh
- excretion toxins into fruit tissue to kill cells by mycelium
- production of extracellular enzymes: to break down substances into low molecular mass compounds to enter fungus cell
- provide building blocks for synthesizing substances and energy for fungus to grow
- production of cutinase: to hydrolyse cutin that overlies epidermis
- penetration of an infection peg through appressorium
- regaining normal size by penetration peg for mycelia of fungus
where do pathogens come from:
a) before harvest
b) during harvest
c) during storage
a) disease in field
b) initial disease and infection
c) pre-cooling delay, storage temp and lesion size, controlled storage
what are ways to provide resistance to infection?
- protection by fruit skin
- wound healing before or after harvest
- phenolic compounds that are toxic to fungi before ripening
- forming inhibitors: post infection inhibitors or phytoalexin
how does lowering temp affect microbial growth?
prolongs lag phase
reduces slope of log phase
what are effects of pre-cooling on microbial growth during storage?
- extends lag phase
- slows down log phase
what are effects of lack of effective pre-cooling on microbial growth?
- lack of lag phase
- accelerated log phase
- extensive spoilage + spore germination
which fungi cannot be stopped by refrigeration without freezing the fruit?
ones capable of growing at sub-zero temps
what does MAP provide indirect disease resistance to?
fruit in good physiological condition
how can you suppress fungus growth with o2 or co2?
what does this do?
lower O2
raise CO2
decreases fungus respiration
what are the 3 major uses of refrigeration for disease control?
- lower temp below min temp for growth of fungi
- lower to a point where the lag phase is not completed before fruit consumption
- kill spores in cold sensitive fungi while they are germinating
how does RH affect germination and direct penetration of fungus pathogens?
how can you prevent this?
high RH = presence of free water on fruit surface = germination can occur more easily = more susceptible to diseases
can prevent by maintaining suitable RH and good air circulation around the commodity
