The diagram shows a cross-section through a leaf affected by a fungal disease. (a) Using the diagram, describe two visible effects of the fungal infection o...

Labelled answer diagram:

(a) Two visible effects of fungal infection on the leaf tissues (from the diagram, any two for [2]):

1. Discolouration and necrosis - the leaf tissue around the infection site turns brown or dark as cells are killed by enzymes and toxins secreted by the fungal hyphae [1].
2. Breakdown of mesophyll cells - the fungal hyphae grow between and through the mesophyll, destroying the chlorophyll-containing cells and leaving areas of dead, collapsed tissue [1].

Also acceptable: fungal hyphae visible growing between cells; spore-producing structures on the leaf surface; death of localised tissue areas.

(b) How fungal spores enter a leaf through natural openings (2 marks):

1. Airborne or splash-dispersed spores land on the leaf surface. In the presence of moisture (dew, rain film), they germinate and produce a germ tube [1].
2. The germ tube grows across the leaf surface until it reaches a stoma. The hypha enters through the stomatal pore and colonises the intercellular spaces of the mesophyll, drawing nutrients from the host cells [1].

(c)(i) The description of dark spots with concentric rings on tomato leaves suggests early blight, caused by Alternaria solani [1]. The concentric ring pattern (target-spot appearance) is a characteristic diagnostic feature of this fungus.

(c)(ii) Three cultural control methods apart from chemical control (any three for [3]):

1. Remove infected material - pick off and destroy affected leaves and collect fallen debris to reduce the source of spores for further spread [1].
2. Use drip irrigation rather than overhead watering, to keep leaf surfaces dry. Alternaria spores require leaf wetness to germinate and infect [1].
3. Adequate plant spacing - space plants correctly to improve air circulation around the foliage, which reduces humidity and shortens the duration of leaf wetness after rain or dew [1].

Also acceptable: grow resistant varieties; practise crop rotation (avoid tomatoes on the same land for at least two seasons); mulch around plants to prevent soil-borne spores splashing onto lower leaves.