Plant Protection: Entomology, Plant Pathology and Weed Science
Plant Protection covers three linked disciplines - entomology, plant pathology and weed science - and the exam tests whether you can identify the causal agent, justify an intervention against a threshold, and stay legal on residues and resistance.
IPM and thresholds
IPM is a hierarchy: prevention (resistant varieties, rotation, sanitation, certified seed, physical barriers, balanced nutrition) → monitoring (traps, visual counts, degree days) → biological control → chemical control last, targeted and rotated.
- Economic threshold (ET): the density at which control prevents the population reaching the economic injury level (EIL).
- EIL = C / (V × D × K): C control cost, V crop value per unit, D damage per pest unit, K control efficiency.
Disease
The disease triangle (susceptible host, virulent pathogen, favourable environment) explains why the same inoculum causes an epidemic one season and not the next. Monocyclic soil-borne pathogens (Verticillium) need long rotations; polycyclic ones (downy mildews, rusts) need protective sprays timed to infection periods. Protectants act before infection; systemic and translaminar products can act after, with higher resistance risk.
Weeds
Selectivity depends on dose, timing, crop and herbicide. Pre-emergence products need a moist soil surface; post-emergence ones need the correct weed growth stage.
Worked example - spray mix and legality
Label: 500 g/L formulation, dose 0.05% in 300 L water/ha, PHI 7 days; harvest planned in 5 days. Product needed = 300 L × 0.05/100 = 0.15 L = 150 mL/ha. Reasoning: the dose and volume are correct, but the PHI (7 days) exceeds the interval to harvest (5 days), so the application is illegal whatever the dose. Do not spray; choose a shorter-PHI product or delay harvest.
Resistance and residues
Rotate by mode-of-action group (IRAC insecticides, FRAC fungicides, HRAC herbicides), not by trade name. MRLs under Reg. (EC) 396/2005 are checked at harvest; the PHI is the on-farm tool that keeps you under them.
Traps
- Spraying at the first insect seen instead of at the threshold.
- Confusing EIL with ET.
- Calling two products of the same MoA group a "rotation".
- Calculating volume correctly but ignoring the PHI.
- Diagnosing a nutrient deficiency (e.g. magnesium in citrus) as disease.