Lecture 18: post harvest pathology

Question 1

define disease

Answer 1

manifestation of the attacking pathogen resulting in the malfunctioning of host ells and their tissues

results from the continued attack and development of pathogens

Question 2

define pathogen

Answer 2

microorganism that can incite a disease on the host

Question 3

define a parasite

Answer 3

organism living on or in another living system (host) an causes the disease by obtaining its food from the latter

Question 4

define an infection

Answer 4

establishment of a parasite (pathogen) within a host plant

Question 5

define pathogenicity

Answer 5

capability of a pathogen to cause the disease

Question 6

define symptom

Answer 6

external or internal reaction or alterations of a plant as a result of a disease

Question 7

define sign

Answer 7

pathogen or parts or products seen on a host plant

Question 8

define rats

Answer 8

softening, discoloration and disinegration of a succulent plant tissue as a result of fungal or bacterial infection

Question 9

describe chlorosis

Answer 9

yellowing of normally green tissue due to chlorophyll destruction or failure of chlorophyll formation

Question 10

what are lesions

Answer 10

visible disease area

Question 11

what biotic or infectious agents causes plant diseases

Answer 11

fungi and bacteria

Question 12

describe how a spore can cause disease

Answer 12

• disperses fungi

Question 13

what abiotic or non-infectious factors causes plant disease?

Answer 13

• too low or high temp
• lack or excess H2O and light
• lack of O2
• air pollution
  etc. ..

Question 14

___ is required for spore germination

Answer 14

Oxygen

Question 15

describe the process of spore germination

Answer 15

1. absorption of moisture
2. spore swelling due to: incr O2 consumption; rapid CO2 evolution; rapid incr in DNA, RNA and protein synthesis
3. germ tube protrudes through spore coat
4. germ tube lengthens and side branches are initiated

Question 16

what are 2 modes of entry of pathogens into fruit?

Answer 16

1. penetrating healthy tissue

2. enter through wounds and bruises

Question 17

______ resists pathogen invasion

Answer 17

plant organs and protective tissues

Question 18

how can you maintain a strong defence mechanism against pathogens?

Answer 18

• growing healthy tissues

Question 19

for a pathogen to be successful, it must… (2)

Answer 19

1. gain access to plant tissue

2. overcome defense response

Question 20

what are the steps of direct penetration of fungi?

Answer 20

1. landing of spore upon fruit
2. germination
3. adhesion of germ tube and appressoria (binding)
4. branching and re-branching of the fruit flesh
5. excretion toxins into fruit tissue to kill cells by mycelium
6. production of extracellular enzymes: to break down substances into low molecular mass compounds to enter fungus cell
7. provide building blocks for synthesizing substances and energy for fungus to grow
8. production of cutinase: to hydrolyse cutin that overlies epidermis
9. penetration of an infection peg through appressorium
10. regaining normal size by penetration peg for mycelia of fungus

Question 21

where do pathogens come from:

a) before harvest
b) during harvest
c) during storage

Answer 21

a) disease in field
b) initial disease and infection
c) pre-cooling delay, storage temp and lesion size, controlled storage

Question 22

what are ways to provide resistance to infection?

Answer 22

• 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

Question 23

how does lowering temp affect microbial growth?

Answer 23

prolongs lag phase

reduces slope of log phase

Question 24

what are effects of pre-cooling on microbial growth during storage?

Answer 24

• extends lag phase

- slows down log phase

Question 25

what are effects of lack of effective pre-cooling on microbial growth?

Answer 25

- lack of lag phase - accelerated log phase - extensive spoilage + spore germination

Question 26

which fungi cannot be stopped by refrigeration without freezing the fruit?

Answer 26

ones capable of growing at sub-zero temps

Question 27

what does MAP provide indirect disease resistance to?

Answer 27

fruit in good physiological condition

Question 28

how can you suppress fungus growth with o2 or co2? what does this do?

Answer 28

lower O2 raise CO2 decreases fungus respiration

Question 29

what are the 3 major uses of refrigeration for disease control?

Answer 29

- 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

Question 30

how does RH affect germination and direct penetration of fungus pathogens? how can you prevent this?

Answer 30

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