Alternaria leaf spot (early blight)

Symptoms

Brown to dark-brown spots appear on the leaves with characteristic concentric, zoned rings resembling a "target board", often surrounded by a yellow halo. The spots enlarge and merge, and the leaves dry out and die, starting with the oldest, lower ones. On fruit, tubers and roots, dark, dry, sunken rot spots form, and the infected tissue becomes covered with a dark-olive, velvety coating of spores. The concentric rings within a spot are the most reliable diagnostic feature — they arise from the alternating, day-and-night growth of the mycelium and are best seen against the light. The picture depends on the host and the fungal species: on potato and tomato (Alternaria solani, "early blight") small dark specks on the lower leaves enlarge into oval, zoned necroses; on brassica vegetables (A. brassicae, A. brassicicola) the spots may be surrounded by a broad chlorotic halo and spread onto the seed pods; on carrot (A. dauci) infection begins at the margins of the older leaves, causing blackening and dieback of the foliage that hampers mechanical harvest. Older spots often die and crumble away in the centre, giving the blade a "shot-hole" appearance, and heavy infection leads to yellowing and premature defoliation, which exposes the fruit to sunscald. On stems and leaf stalks elongated, dark, sunken streaks form, and in seedlings the fungus can cause damping-off at the base of the shoot.

Treatment

Removing infected leaves and plant residues and applying fungicide sprays preventively, especially in warm and humid periods. Wetting the leaves and working among wet plants should be avoided. Once the first spots are noticed, it is best to intervene early, while only the lower leaves are affected — remove and destroy them (do not compost them in a backyard heap), and as disease pressure rises, start a fungicide protection programme every 7–14 days, taking care to cover thoroughly the underside of the blade and the lowest leaves as well. Protection combines contact products that form a barrier on the leaf (copper compounds) with systemic and translaminar fungicides from different groups — strobilurins (QoI), triazoles and SDHI. The groups must be rotated, because Alternaria quickly develops resistance, especially to strobilurins (QoI) — the same group should not be repeated in successive treatments. In parallel it is worth opening up the canopy, limiting lingering moisture (watering at the base, giving up overhead sprinkling), and harvesting fruit and tubers in dry weather, without the injuries through which the fungus enters. Note: some formerly used contact fungicides have been withdrawn from the market in the EU, so always verify the choice of active substance against the current register of products approved for the given crop.

Prevention

Crop rotation, healthy seed, balanced fertilisation to limit deficiencies that weaken plants, an airy position, watering at the base, removing crop residues and growing less-susceptible cultivars. Crop rotation with a 3–4-year break in growing host plants on the same field, combined with deep ploughing-in or removal of residues, significantly lowers the reservoir of spores overwintering in the soil and on debris. Because the fungus is also carried on seed, certified, healthy material is crucial, and for carrot and brassica vegetables — thermal disinfection of seed in warm water (about 50°C). Balanced fertilisation has a double importance here: a shortage of nitrogen and potassium clearly increases susceptibility, but an excess of lush leafy growth hampers drying of the canopy — so one aims for a balance. Plants are set at a wider spacing that ensures air movement and rapid drying of the leaves after dew, watered at the base (preferably in the morning, not the evening), and the site is kept clear of self-sown seedlings and weeds that could be a reservoir of infection. Regular monitoring of the lower leaves also helps, as does — where available — the choice of cultivars with increased tolerance.

How to tell it apart from look-alikes

Control — organic and chemical methods

We give active-ingredient groups and modes of action, not brand names — specific products and their approval vary by country and are sometimes withdrawn. Always check and follow the current label.