Management of late blight, the most aggressive disease in potato crops

Rapid spread and high destructive potential make late blight the most important and aggressive disease in potato crops, with damage that can result in total loss in a few days. To correctly manage the disease, the producer must adopt integrated measures. The sequential application of contact fungicides from emergence, with subsequent use of products with systemic activity in the vegetative growth and tuberization phases, is an important strategy, mainly to prevent the selection of resistant pathogens.

The biggest limitation to potato production, mainly in the central region of Rio Grande do Sul, is the occurrence of late blight or blight, caused by Phytophthora infestans. The rapid spread and high destructive potential characterize this disease as the most important and aggressive disease in potato crops worldwide. When control measures are not adopted correctly, late blight epidemics can destroy the entire production field in a few days.

Late blight can occur at any stage of crop development, with the potential to severely affect the entire aerial part of the plant such as leaves, petiole and stem, and in some cases the tubers. In general, symptoms vary depending on temperature, humidity, light intensity and host resistance.

On the leaves, symptoms begin in the form of small spots of light green to dark green color. It is also possible to observe a waterlogged halo that separates the lesion from healthy tissue. In conditions of high humidity, these spots increase in size and acquire an irregular shape, with a waterlogged appearance and a dark, brownish or black color. The affected tissues then become necrotic, presenting a burning appearance (Figure 2). Lesions can advance to the petioles and stem, causing the plant to die. In conditions of high humidity, fruit formation of the pathogen is observed on the abaxial side of the leaves. In conditions of low humidity, the growth of lesions stops and the tissue becomes brittle.

On the petiole and stem, the lesions are similar, which can encircle the entire organ and cause its death, while on the tubers, brown spots may appear on the epidermis. In these, the fungus causes hard, dark rot with defined edges, which reach approximately 1,5 cm in depth.

 

Etiology

In Brazil, late blight is caused by the oomycete Phytophthora infestans, occurring in practically all regions where potatoes and tomatoes are cultivated. When humidity and temperature conditions are favorable for the development of the pathogen, it produces a large number of mobile spores known as zoospores, responsible for infections and epidemics. Zoospores can travel through a water film in the soil and cause new infections. On the other hand, when temperature and humidity conditions are unfavorable to the pathogen, it produces resistance structures known as oospores, which play an important role in the survival of the pathogen, until conditions become favorable again. They can remain viable in the soil for three to four years.

Epidemiology

Conditions of high relative humidity and low temperature, generally below 20°C, are the factors that most contribute to the occurrence of the disease. Nonetheless, P. infestans grows and produces zoospores abundantly at relative humidity close to 100% and temperatures between 15ºC and 25°C. At temperatures above 30ºC, the progress of the disease is slow or even paralyzed. Leaf wetness caused by rain or irrigation and dew directly affects the infection and, consequently, the rate of progress of the disease in the field. Therefore, planting susceptible cultivars or those with low levels of resistance, associated with periods with low temperatures and high humidity, is a highly favorable factor for late blight, which can cause destruction and total loss of the crop in a few days.

The pathogen survives mainly in crop residues and diseased tubers, being spread by rain, strong winds and contaminated agricultural implements. Tomato crops in the final stage of production can also eventually host the pathogen, serving as a source of inoculum for later potato crops.

 

Disease management

  Late blight can be successfully controlled through a combination of sanitary measures, resistant cultivars and the application of fungicides.

Genetic control of late blight, despite being the most advantageous option for producers, as it is more economical and easy to use, is not sufficient to control the disease when environmental conditions are very favorable to the pathogen. Therefore, the planting of resistant cultivars is limiting, as most commercial cultivars in Brazil are susceptible (Ágata, Cupido, Mondial, Bintje) or tolerant (Monalisa, Atlantic, Astetrix, Vivaldi, Baronesa). Another difficulty refers to the high genetic variability of the pathogen aggravated by the lack of knowledge of the behavior of cultivars against isolates from each region. Even tolerant varieties should be sprayed regularly with fungicides to eliminate, as far as possible, the possibility of infection caused by the pathogen.

Currently, potato producers have a large number of fungicides registered with the Ministry of Agriculture, Livestock and Supply (Mapa) to control late blight, including contact products and with different levels of systemic activity.

Contact fungicides must be applied before the start of infection to be effective, and are recommended throughout the entire crop cycle. These fungicides have a protective action and, therefore, must be applied periodically to promote coverage of the entire aerial part of the plants, as they are not translocated. These are products that remain on the leaf surface, but are subject to removal by the action of rain and irrigation. The protection period for these fungicides varies from four to eight days. In general, spraying to renew plant protection must be repeated at intervals of four to seven days in rainy periods or periods of rapid vegetative development of the crop, and seven to ten days in dry periods. Systemic fungicides have curative action and are translocated through the plant's vascular system, with the characteristic of being distributed throughout the plant as a whole. They are quickly absorbed (30 minutes on average) and protection periods of ten to 14 days, on average.

The main contact fungicides recommended for controlling late blight in potatoes are: cuprics, mancozeb, metiram, chlorothalonil, zoxamide + mancozeb, fluazinam and capitan, and the main systemic fungicides are: propamocarb, pyraclostrobin, propineb, metalaxyl-M in mixture with mancozeb and chlorothalonil. Other fungicides registered for the control of P. infestans can be consulted via the website: http://extranet.agricultura.go... (Agrofit).

It is worth mentioning that the application of fungicides must be carried out sequentially using contact products from emergence, with subsequent use of pesticides with systemic activity in the vegetative growth and tuberization phases. This alternation of active ingredients and modes of action is extremely important to prevent the selection of fungicide-resistant pathogens.

Although the use of fungicides is the most efficient way to control the disease, some cultural measures contribute to controlling the disease, such as: the use of healthy seed potato tubers; avoid planting in contaminated areas or lowlands subject to water accumulation; prioritize planting at times less favorable to the occurrence of the disease, that is, in the hottest periods of the year; carry out irrigation early in the morning, so that the leaves dry before nightfall; destruction of inoculum sources, and crop rotation with grasses or other non-solanaceous plants.

 

 Potatoes in Brazil

Potatoes, among vegetable crops, have the largest cultivated area, ranging from 150 thousand to 180 thousand hectares. According to the Brazilian Institute of Geography and Statistics (IBGE), estimated national production for 2012 is 3,66 million tons, a mark already surpassed last year and in 2008.