disease → modes-of-spread
Once a pathogen is present, disease severity depends largely on how it spreads.
Modes of spread determine: - How fast disease moves through a crop - Which management actions are effective - Why some outbreaks escalate suddenly
Spread is often underestimated because it is invisible until symptoms appear.
It is important to distinguish between:
Many management actions fail because they focus on infection while ignoring spread.
Splash spread occurs when water droplets move inoculum short distances.
Common sources: - Rainfall - Overhead irrigation - Condensation drip - Runoff along leaves or benches
Key features: - Highly localised - Strongly influenced by irrigation timing - Rapid increase after initial infection
Splash spread explains why: - Lower canopy symptoms appear first - Disease clusters rather than spreads evenly - Irrigation method matters more than volume
Airborne spread involves spores or particles moving via airflow.
Sources: - Natural wind - Fans and ventilation - Thermal convection in glasshouses
Key features: - Long-distance movement - Rapid distribution - Strongly influenced by humidity and temperature
Airborne pathogens often: - Appear suddenly across large areas - Exploit brief favourable windows - Produce widespread, uniform symptoms
Contact spread occurs through physical transfer.
Common routes: - Hands and gloves - Tools and equipment - Clothing - Plant-to-plant contact
Contact spread is often underestimated but is significant in: - Dense canopies - Propagation areas - Repetitive handling systems
Small lapses can move inoculum efficiently.
Some pathogens are spread by living organisms.
Examples: - Insects - Mites - Nematodes - Humans (unintentionally)
Vector spread often bypasses environmental barriers, making: - Infection more likely - Timing harder to predict - Control more complex
Once secondary spread begins:
This is why early outbreaks feel manageable, then suddenly overwhelming.
Modes of spread determine how quickly inoculum pressure builds.
Understanding spread helps prioritise interventions.
Effective disease control limits spread by:
Slowing spread often matters more than preventing the first infection.