Dreaded villain: tomato late blight

Late blight, caused by Phytophthora infestans, is a highly aggressive disease in tomato crops, capable of decimating entire crops in a short space of time.

17.01.2018 | 21:59 (UTC -3)

Late blight, caused by Phytophthora infestans, is a highly aggressive disease in tomato crops, capable of decimating entire crops in a short space of time. Lacking cultivars with good levels of resistance to the pathogen, Brazilian producers need to use integrated measures, preferably preventive, with daily and rigorous monitoring of cultivation areas, in addition to adopting chemical control with specific fungicides for each stage of management.

One of the major limitations for tomato production in the field is the incidence of diseases, which occur with greater intensity in rainy seasons, responsible for losses that compromise the entire production. Among the diseases, late blight or honeydew, caused by Phytophthora infestans, stands out for causing serious damage to the crop, especially in regions with a mild climate and high relative humidity.

The disease is feared by most tomato growers, due to its aggressiveness. When environmental conditions are very favorable to the development of the pathogen, the crop can be completely destroyed in a few days, due to plant defoliation, branch death and fruit rot.

Despite being a widely studied disease, its control is still difficult. Since the 1990s, control of late blight has been carried out through the adoption of cultural practices and the application of systemic fungicides, since there are tomato cultivars with satisfactory levels of resistance to the disease. Some tomato producing regions, where environmental conditions are very favorable to the occurrence and epidemic of the disease, have become productively unsustainable at certain times of the year, due to the high costs of applying fungicides to control late blight.

The symptoms

Late blight can occur at any stage of tomato development and severely affects all organs in the aerial part of the plant. The first symptoms usually occur in the upper half of the plant. On the leaves, they begin in the form of small spots of pale green color and an undefined shape. Subsequently, the lesions rapidly increase in size and affect a large part of the leaf blade. The affected tissues then acquire a pale brown color, wither and become necrotic with a burning appearance, which is why the disease is called late blight.

Lesions can also occur in a similar way on the stem, petioles and rachis of the tomato plant, in the form of dark lesions, generally superficial, brittle, which can result in the death of the portion above the lesion.

On the fruits, the lesions appear as dark brown-brown spots, with an oily appearance and firm consistency, which can increase in size and extend over the entire surface of the fruit, causing hard rot, without causing it to fall. Over time, infected fruits may become soft due to contamination with opportunistic microorganisms. In conditions of high humidity, the formation of mycelium and fructification of the pathogen occurs on stems, petioles, fruits and on the underside of leaves.

Etiology

Tomato late blight is caused by the oomycete Phytophthora infestans, occurring in practically all regions where tomatoes and potatoes are cultivated. The fungus presents very high variability, and numerous physiological races have already been described, which makes it difficult to develop and establish cultivars with satisfactory levels of resistance to the disease.

When humidity and temperature are favorable for development, the pathogen 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 are unfavorable for its development, the pathogen produces resistance structures known as oospores, which play an important role in its survival, until conditions become favorable again. Oospores are capable of surviving in the soil in the absence of a host plant, constituting an initial inoculum, with an important role in the epidemiology of the disease by contributing to the early onset of the epidemic in the field. 

Epidemiology

The most important climatic factor for the onset of the disease is moisture from rain or irrigation and dew, so much so that in tomato cultivation it is possible to observe outbreaks of late blight even in the hottest months of the year, as long as the presence of free water is sufficient. on the leaves or stems and cold nights. P. infestans grows and produces zoospores abundantly at relative humidity close to 100% and temperatures between 15ºC and 25°C. Therefore, in locations and growing seasons with a mild climate and high relative humidity, the amount of inoculum is much greater, which leads to more severe epidemics in the field after several successive cycles. In a favorable environment, the pathogen can complete an infection cycle in four to five days.

The pathogen survives mainly in tomato and potato crop residues, such as infected leaves, stems, fruits and tubers, and is spread by rain, strong winds and contaminated agricultural implements. Tomato and potato crops in the final stages of production can also eventually host the pathogen, serving as a source of inoculum for subsequent tomato crops or nearby crops.

The seeds, although in low occurrence, can also serve as a source of inoculum for the disease, promoting its spread through infected seedlings.

Disease management

The integrated use of management practices is necessary for successful control of late blight, which includes cultural preventive measures and application of fungicides. Currently, there are no tomato cultivars with good agronomic characteristics and long-lasting resistance to P. infestans. What can be seen in practice is that there are different levels of susceptibility to late blight in commercial cultivars, but all are attacked by the disease. 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.

Preventive methods should be prioritized whenever possible to be more successful in controlling the disease. The first step is choosing the area and planting time. Planting should be avoided in soils known to be contaminated by the fungus, in lowlands, subject to the occurrence and persistence of fog for long periods, poorly drained soils or areas close to old contaminated tomato and potato crops. The ideal is to plant in warmer and less rainy seasons whenever possible, as one of the ways to discourage the establishment of the pathogen and the progress of the disease. It is important to remember that planting healthy seedlings is essential and irrigation should preferably be drip to avoid causing leaf wetting. After the last tomato harvest, it is recommended to immediately incorporate the crop residues into the soil, so that there is sufficient time for their decomposition, in order to eliminate sources of inoculum for successive tomato plantings. In contaminated areas, it is recommended to rotate crops with grasses or other non-solanaceous plants.

To ensure the efficiency of chemical control, it is necessary to carry out daily monitoring of the crop in order to check the first symptoms of the disease. The right time to apply the fungicide is key to controlling the disease. Applications of contact fungicides must be started before the disease appears, 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 days to eight days. In general, spraying to renew plant protection must be repeated at intervals of four days to seven days in rainy periods or periods of rapid vegetative development of the crop, and from seven days to ten days in dry periods.

Systemic fungicides have curative action and should therefore be applied after the first symptoms are detected. These pesticides are translocated through the vascular system, with the characteristic of being distributed throughout the plant as a whole. They are quickly absorbed, in around 30 minutes, and have protection periods of approximately ten days to 14 days. It is worth mentioning that the application of fungicides must be carried out sequentially with contact products from emergence, with subsequent use of products with systemic activity in the vegetative growth and fruiting phases. This alternation of active ingredients and modes of action is extremely important to prevent the selection of fungicide-resistant pathogens.

Currently, tomato growers 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. The main contact fungicides recommended for controlling late blight in tomato are: cuprics, mancozeb, chlorothalonil, fluazinan, propineb, Captan, mandipropamid, zoxamide and cyazofamide, and the main systemic fungicides are: cymoxanil, fenamidone, dimethomorph, famoxadone, metalyl -M and benalaxyl. Some of the fungicides already contain mixtures of active ingredients with protective and systemic action. Fungicides registered for the control of P. infestans can be consulted via the website: http://extranet.agricultura.go... (Agrofit, 2012). For fungicide applications, the producer must strictly follow the manufacturer's recommendations regarding the dose, number and interval of application, the volume of the product and the mixture to be applied, the withdrawal period and the withdrawal period. It is worth mentioning that the use of personal protective equipment (PPE) is essential to protect the applicator.

 

Click here to read the article in issue 78 of Cultivar Hortaliças e Frutas. 

 

 

 

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