3 Fig. 3.1 shows a tomato plant affected by a fungal disease. The leaves have dark brown spots surrounded by yellow areas, and some lower leaves have wilted...

Labelled answer diagram:

(a) Three visible symptoms of the fungal disease on the tomato plant in Fig. 3.1:

1. Dark brown spots or lesions on the leaves. [1] These are areas where the fungal pathogen has colonised the leaf tissue, killing the cells and producing the characteristic dark discolouration.
2. Yellow (chlorotic) areas surrounding the brown spots. [1] The yellowing occurs because the fungus disrupts the leaf's ability to photosynthesise in the tissue adjacent to the infection site. Enzymes and toxins released by the fungus degrade chlorophyll in the surrounding cells.
3. Wilting of the lower leaves. [1] The lower leaves are typically the first affected because fungal spores splash up from the soil onto them. As the infection progresses, the damaged vascular tissue can no longer transport water efficiently, and the leaf loses turgor and wilts.

Other acceptable symptoms include curling or drying of leaf edges, and discolouration or rotting of fruit.

(b) How fungal diseases spread from one plant to another in a crop field:

1. Wind dispersal of spores. [1] Fungi produce vast numbers of lightweight spores that are released into the air from infected tissue. These spores are carried by wind currents to neighbouring healthy plants, where they land on leaf surfaces, germinate, and start new infections.
2. Splashing rainwater. [1] When rain hits infected leaves or the soil surface containing fungal spores, it creates small droplets that splash spores onto the foliage of adjacent plants. This is particularly effective for diseases that infect the lower canopy.
3. Contaminated soil and plant debris. [1] Many fungal pathogens survive in the soil or on dead plant material (crop residues) left in the field from previous seasons. When new plants grow in contaminated soil, the fungus colonises them from below, and spores produced on the debris serve as a source of infection for the new crop.

(c) Four methods a farmer could use to control fungal diseases in tomato crops:

1. Apply fungicide sprays at recommended intervals. [1] Chemical fungicides (protectant or systemic) kill the fungus or prevent spore germination on leaf surfaces, reducing the spread and severity of infection.
2. Remove and destroy infected plant material. [1] Pruning off diseased leaves and stems and burning or burying them reduces the source of spores in the field, limiting further spread to healthy plants.
3. Use resistant or tolerant varieties. [1] Planting tomato cultivars that have been bred for resistance to common fungal pathogens reduces the likelihood and severity of infection without relying solely on chemical control.
4. Space plants adequately to improve air circulation. [1] Wider spacing allows air to flow between plants, reducing the humidity around the foliage. Fungi thrive in warm, humid conditions; improved ventilation dries leaf surfaces faster and makes them less hospitable for spore germination.

Other acceptable methods include practising crop rotation with non-host crops and avoiding overhead irrigation to keep leaves dry.

(d) Why crop rotation is effective in reducing disease in subsequent growing seasons:

1. Many fungal pathogens are host-specific and cannot survive in the soil without a suitable host plant to infect. [1] When the farmer grows a different, unrelated crop in the following season, the fungus has no living tissue to colonise and its population declines.
2. Growing a different crop breaks the disease cycle, preventing the build-up of spore populations in the soil from season to season. [1] By the time the host crop (tomato) is planted again, the pathogen inoculum in the soil has been significantly reduced, resulting in lower disease incidence.