Plant Pathology

Step 1: Identify the historic disease.
The mid-19th century Irish potato famine is historically linked to late blight of potato.
Late blight devastates foliage and tubers rapidly under cool, wet conditions.
Step 2: Name the causal organism.
Late blight is caused by Phytophthora infestans, an oomycete that spreads through sporangia and zoospores.
Step 3: Eliminate distractors.
Alternaria solani causes early blight, not the famine-scale late blight.
Synchytrium endobioticum causes wart disease of potato, a quarantine problem but not the famine agent.
Erwinia carotovora (now Pectobacterium) causes soft rot, mainly during storage and wounds.
Step 4: Conclusion.
Hence, the Irish potato famine was caused by Phytophthora infestans.

Step 1: Recall host–pathogen pairs for common powdery mildews.
Barley powdery mildew is classically associated with Erysiphe graminis var. hordei, a host-specific form that attacks cereals.
Pea powdery mildew is commonly attributed to Erysiphe polygoni in standard horticulture texts for legumes.
Grape powdery mildew is caused by Uncinula necator, the well-known agent in vineyards worldwide.
Apple powdery mildew is caused by the organism historically recorded as Sphaerotheca or Podosphaera leucotricha; in the given list it appears as Cercosphaera leucotricha and represents the same disease entity for matching.
Step 2: Perform the matching.
A (barley) → IV (Erysiphe graminis var. hordei).
B (peas) → III (Erysiphe polygoni).
C (grape vines) → II (Uncinula necator).
D (apple) → I (Cercosphaera leucotricha).
Step 3: Conclusion.
Therefore the correct mapping is A - IV, B - III, C - II, D - I, which corresponds to option (4).

Step 1: About phytopathogenic bacteria.
These are bacteria that cause diseases in plants.
They are different from human/animal pathogens.
Step 2: Key characteristics.
- B: They do not infect animals → Correct, restricted to plants.
- C: They are mostly rod-shaped and flagellate → Correct.
- D: Almost all plant pathogenic bacteria are Gram-negative → Correct.
- A: They are saprophytes → Wrong, phytopathogenic bacteria are parasitic, not saprophytic.
Step 3: Conclusion.
Thus, the important characteristics are B, C and D only.