Causal agent(s) and transmission
Potato late blight is caused by Phytophthora infestans which is not a fungus but a water mould also known as “oomycete”. It produces mycelium which can be of two different type of strains: A1 and A2, that are of opposite sexual compatibility. The mating of the two sexually compatible strains, on a potato plant, may lead to the formation of oospores (photo 1). The latter are resistant organs which can survive in the soil for several years. Until now, this has been a rare event in Europe.
More frequently, the cycle of Phytophtora infestans is asexual. In winter, it normally survives in the form of asexual mycelium in tubers left in the field (ground keepers), in waste piles and in storage. In the spring, this surviving mycelium produces sporangia which are spread by wind and rain, infecting new healthy plants and crops. Under conditions of high humidity, one cycle normally takes 4 to 6 days at an average temperature of 15°C. After the primary infection, subsequent infection cycles can occur and be responsible for a very rapid and destructive epidemic on the foliage.
Later in the growing cycle of the potato, newly formed tubers are directly infected in the field: sporangia formed on the aerial parts of the plants are carried by rainwater into the soil and infect the tubers.
Potato and tomato plants are the unique hosts of this pathogen.
Occurring worldwide, potato late blight remains the most difficult disease to control in potato production. Late blight is a threat wherever potatoes are grown andit can be particularly serious in crops grown in temperate, cool and humid climates. Under high disease pressure, optimal late blight control requires repeated and preventive, expensive chemical treatments. If left uncontrolled, the economic result can be a complete yield loss.
Under organic potato production, the control of this disease is one of the most challenging procedures due to the lack of authorised fungicides (only copper-based products).
Symptoms on foliage
On the upper side of the leaves, small discoloured spots may be observed; these turn brown and are surrounded by a light-green to yellow halo (photo 1).
On the underside of leaves and in wet conditions, the asexual fructifications of P. infestans (sporangiophores and sporangia) appear around the spots giving a characteristic white felting. An increase in the number of spots, their expansion and subsequent desiccation can rapidly destroy the foliage (photo 2).
Brown lesions, sometimes necrotic, can be observed on the stems (photo 3) and on the apical parts of the canopy (photo 4). Under moist conditions, all the plant organs can be infected including the berries.
Grey mould due to Botrytis cinerea
Symptoms on foliage due to B. cinerea may be confused with late blight symptoms, caused by P. infestans. Cool and wet conditions greatly enhance grey mould infection due to B. cinerea (photo 5). However due to the saprophytic nature of the fungus, it tends to infect weak and senescent plant parts.
Chemical treatment usually controls the problem, which rarely has an economic impact on potato crops.
Symptoms on tubers
Externally, the tubers display unclearly delineated, brown or purplish-grey spots (mottling), which may be slightly depressed (photos 6 and 7).
When infected tubers are cut open, they contain areas of rust-coloured mottled flesh just under the epidermis (photo 8), although the decay may extend to the centre of the tuber.
Other pathogens can then develop and cause damp rot if the harvested tubers are not well- ventilated.
Factors particularly favourable for potato late blight include temperatures in the range of 15-20°C, a high level of humidity and excessively dense foliage.
Depending on the temperature, sporangia germinate directly or release biflagellate zoospores. In optimum conditions, the length of the infective cycle is 3 to 5 days; this explains the explosive nature of the epidemics and the extremely rapid development of the disease.
The susceptibility of potato cultivars is a major factor in the epidemic development of potato late blight, although most cultivars are (moderately to very) susceptible to the disease.
The control of potato late blight must combine various preventive measures:
- prefer resistant or tolerant cultivars, when available;
- use healthy seed tubers;
- destroy waste piles and groundkeepers in the adjacent crops, which can serve as a primary inoculum reservoir;
- ensure effective ridging and nitrogen fertilisation;
- chemical preventive protection of the foliage should be applied during all the growing period whenever required by the disease pressure. Numerous fungicides are effective against potato late blight and must be used according to their recommendations and their mode of action: contact fungicides (e.g. dithiocarbamates, copper-based products and others), translaminar or systemic fungicides. Care should be taken concerning the risk of the resistance of P. infestans natural populations to certain fungicides, in particular with systemic fungicides used in fields where late blight is already present;
- during the growing period, it is highly recommended to take account of the local weather forecast and of blight warnings issued by the blight monitoring services to adapt the fungicide protection. Unless the environment of a potato crop can be regularly inspected to check for the absence of the disease, it is advisable to subscribe to the local specialised warning system or to have a personal Decision Support System for field advice; this tool is available in most European countries;
- in order to protect the crop until tubers are harvested, fungicide protection must be carried out up to the time of haulm destruction.