Potato and tomato late blight

Symptoms

Dark-brown, spreading spots on the leaves (often with a lighter margin and a whitish coating on the underside in humid conditions), brown streaks on the stems, and hard, brown rots on tomato fruit. In wet, warm summers it develops with lightning speed and can destroy a crop in a few days. The first symptoms are small, watery, grey-green spots that usually appear on the edges and tips of the leaves and within a few days enlarge into extensive, brown-black, dying areas - the spots do not stop at the leaf veins and cover the whole blade. The characteristic whitish, delicate coating on the underside of the leaf (this is the pathogen's sporulation - the sporangiophores) forms at the edge of the spot in high humidity, most often in the morning or after rain. The stems and leaf stalks become covered with brown, darkening streaks, at which the shoot turns brittle and easily broken. On potato tubers, bluish-rusty, sunken discolourations of the skin appear, beneath which the flesh turns into a rust-brown, granular, hard rot penetrating irregularly into the interior - often secondarily colonized by bacteria, which develops into a foul-smelling wet rot. On tomato fruit (including green, unripe fruit) hard, greasy, brown, irregular spots form. Under favourable conditions (cool nights, warm and humid days) an infected plantation can 'burn up' within a few days, giving off the characteristic smell of rotting haulm.

Treatment

Immediately remove infected leaves and fruit; in organic growing the options are limited - prevention is key. Under high disease pressure, copper-based or fungicidal preparations are applied preventively. It must be remembered that in controlling late blight fungicides act above all PREVENTIVELY (protectively), not curatively - they should be applied before the disease appears or at the first, isolated spots, because once a plantation is overrun no preparation will 'undo' the damage. Infected leaves, shoots and fruit are removed and destroyed (burn them or dispose of them off the property - do not compost them and do not leave them on the bed). In potato growing, when the haulm is heavily infected, it is crucial to destroy the haulm (by mowing, mechanically or - in commercial production - by desiccation) 2-3 weeks before harvest and to lift the tubers only after the skin has dried and in dry weather - this protects them from infection by spores washed off the leaves into the soil. Only healthy, dry tubers are suitable for storage. In tomato growing, infected fruit and leaves are removed, ventilation is improved, and plants under cover are protected from wetting. Chemically, contact fungicides (copper-based) are used preventively, while in commercial production penetrant/systemic products are always applied in a mixture with a contact one and with alternating use of different groups, in order to limit the risk of the pathogen developing resistance.

Prevention

Airy planting spacing, watering at the base (not onto the leaves), growing tomatoes under cover (a roof protects against rain), crop rotation, resistant varieties, and avoiding the proximity of potatoes. The basis of protection is healthy starting material and the removal of infection sources: plant certified, healthy seed potatoes, systematically remove potato volunteers, and do not leave heaps of tuber waste (this is the main, earliest spring source of spores). Earth up potatoes carefully and high - a thick layer of soil over the tubers makes it harder for spores to reach them. Choose cultivars with increased resistance as well as early cultivars, which finish their growth before the peak of disease pressure (the second half of summer). Keep spatial separation of potato and tomato (the same pathogen passes between them) and use crop rotation. Fertilize in a balanced way, without an excess of nitrogen, which produces lush haulm that stays wet for a long time. Water at the base and in the morning, so that the leaves have time to dry, and grow tomatoes under cover that protects them from rain and dew. In regions and years of high risk it helps to follow warning-service bulletins (critical periods - sequences of days with high humidity and moderate warmth) and to carry out the first protective treatment preventively, before symptoms appear.

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.