Quick Checks

Partial credit: The claim is partly correct — bark does protect woody stems from fungal entry, acting as a physical barrier. However, the reasoning is wrong: spore size is not the main reason. Bark excludes pathogens because it is made of dead, thick-walled cells that are difficult to penetrate enzymatically. Rose black spot primarily affects leaves because spores land on leaf surfaces and germinate in moist conditions, penetrating the thinner cuticle — not because spores are "too large" for bark.

Model answer: A virus (such as TMV or cucumber mosaic virus) is most likely. The mosaic/mottled pattern of light and dark patches is the characteristic symptom of viral infection — viruses disrupt chloroplast development causing uneven chlorophyll distribution. The absence of surface growth (e.g., no white powder, no spots) argues against a fungal infection, which typically produces visible mycelium or spores. Bacterial infection more commonly causes wilting, soft rot, or water-soaked lesions rather than mosaic patterning. The farmer should also check for aphid infestation, as aphids are vectors that transmit many plant viruses.

Model answer: Fungal spores require water to germinate — in wet climates water sits on leaf surfaces for longer, increasing germination risk. A thick waxy cuticle is hydrophobic (water-repelling), so water droplets and fungal spores roll off the surface rather than remaining in contact with the leaf. This reduces the time spores have to germinate and begin penetrating the leaf. Even if spores do land, a thicker cuticle takes longer for the fungus to penetrate enzymatically, giving the plant more time to activate chemical defenses. A thinner cuticle allows faster water retention and easier spore penetration, meaning infection is more likely and spreads more rapidly.