Holding the status of the most important and destructive disease in tomato cultivation, late blight relentlessly attacks leaves, petioles, stems and fruits. Rational and integrated measures are necessary to prevent their incidence or minimize losses in affected areas.
Late blight, caused by the oomycete Phytophthora infestans (Mont) de Bary, is the most important and destructive disease of tomato crops, and can drastically affect leaves, petioles, stems and fruits.
On the leaflets, symptoms are initially characterized by spots of varying size, light or dark green color and moist appearance. As they evolve, they become dark brown, necrotic and irregular. On the petioles and stems the spots are moist, dark brown, elongated and ringed. On fruits, the disease causes hard rot, characterized by irregular, deformed, deep, dark brown lesions. On the underside of the leaves and other affected organs, a velvety white growth is frequently formed, on or around the necrotic tissue, consisting of sporangia and sporangiophores of the pathogen. The disease can also cause young plants to fall over or cause seed germination failure, thus reducing the stand during the seedling production phase.
Late blight is favored by temperatures ranging from 12ºC to 25ºC and daily leaf wetness for more than 12 hours. Sporangia germinate directly when temperatures range from 18ºC to 25ºC, or they can produce biflagellate zoospores when they are in the range of 12ºC to 16°C. Each sporangium gives rise to an average of eight zoospores, which significantly increases the amount of inoculum and consequently the severity and destructive potential of the disease. The penetration of the pro-mycelium, resulting from the germination of sporangia or encysted zoospores, is direct into the plant tissue, with the formation of an appressorium. Tissue colonization is rapid, with the incubation period varying from 48 to 72 hours.
Regarding humidity, late blight is favored by periods of leaf wetness exceeding 12 hours and environments of mist and light rain. In some situations, the altitude associated with the presence of dew and the drop in nighttime temperatures are sufficient for important epidemics of the disease.
Among the factors that favor the occurrence of severe late blight epidemics in our growing conditions, the following stand out: the lack of resistant cultivars and hybrids, favorable climatic conditions, sequenced plantings and the existence of inoculum throughout the year.
The spread of late blight occurs mainly via infected seeds and seedlings, wind, rainwater or irrigation, etc.
In addition to the tomato plant, P. infestans can affect potato crops (Solanum tuberosum L.), pepper (Capsicum annuum L.), eggplant (Solanum melongena L.), petunia (Petunia hybrida) and invasive plants such as: fig tree of hell (datura stramonium L.), white picão (Galinsoga parvifora Cav), viola string (Ipomea purpurea L.), false cloak jewel (Nicandra physaloides L.), joá de capote (physalis angulata L.), black maria (Solanum americanum L.), wonder (jalapa mirabilis L) and Nicotiana benthamiana Dom.
The high destructive potential and rapid evolution of late blight in the field make it mandatory to adopt rational and integrated control measures, with the aim of guaranteeing the production and sustainability of tomato cultivation.
The main measures recommended to control damping-off in the production of tomato seedlings (one of the effects caused by late blight) are to use certified seeds treated with fungicides, use a pathogen-free substrate; eliminate and destroy diseased seedlings; avoid excess nitrogen fertilization in the substrate; use good quality water, avoid excessive irrigation; use countertops with open mesh to reduce humidity levels; disinfect countertops and trays with 4% formaldehyde or 5% sodium hypochlorite.
As for tomato planting sites, it is recommended to avoid areas subject to moisture accumulation, limited air circulation and close to crops at the end of their cycle. These measures aim to prevent favorable conditions and the presence of inoculum in new fields.
It is also not recommended to plant successive tomatoes and/or other nightshades. Crop rotation is essential to reduce the inoculum potential in cultivated areas.
Dense plantings should be avoided, as they encourage poor air circulation and the accumulation of moisture between the plants. Adopt driving systems that favor handling. In tutored crops, vertical conduction can reduce the occurrence of late blight by favoring air circulation between plants. In low-lying crops, choosing materials with an open architecture can facilitate disease management through greater air circulation between plants and better penetration of fungicides into the foliage.
It is also recommended to eliminate remaining fruits in the field and volunteer plants, originating from fruits left in the field at harvest and which can be an important source of inoculum for new crops. They must be eliminated by the use of herbicides or mechanical methods. The complete elimination of fruits after harvest prevents the emergence of volunteer plants, as well as preventing infected fruits from producing inoculum that can be disseminated to new crops.
Another important aspect is controlled irrigation. Avoiding long periods of leaf wetness is essential for managing late blight. To this end, sprinkler irrigation, at night or in the late afternoon, must be eliminated; as well as minimizing the time and reducing the frequency of watering in fields with symptoms.
Balanced fertilization is also important. High nitrogen levels result in tissues that are more tender and susceptible to late blight. Excess nutrients also allow excessive growth of the aerial part and consequently greater accumulation of moisture in the foliage. Adequate levels of phosphorus, calcium and boron can reduce the incidence and severity of late blight.
Correct management of invasive plants helps combat late blight. In addition to competing for space, light, water and nutrients, these vegetables make it difficult for moisture to dissipate and air circulation in the foliage. In some cases, they can also be alternative hosts for P. infestans.
Cleaning and disinfecting equipment used in affected crops, eliminating and destroying diseased fruit and discards are important to eliminate possible sources of inoculum.
Proper storage aims to promote good temperature, humidity, air circulation and hygiene conditions during fruit storage. Constant inspection of the culture to identify possible initial outbreaks of the disease and speed up decision-making is also important.
The use of fungicides must follow all manufacturer recommendations regarding dose, volume, interval and number of applications, safety, use of personal protective equipment (PPE), storage, disposal of packaging, etc.
To avoid cases of resistance to specific fungicides (mobile in the plant) it is recommended that they be used alternately or formulated with non-specific products (contact). It is important to avoid repetitive use of fungicides with the same mechanism of action and not to carry out curative applications in situations of high disease pressure.
Table 1 describes the main fungicides registered for the control of tomato late blight in Brazil.
In addition to cultural practices such as planting at the appropriate time; use of healthy seeds and seedlings; correct choice of area; balanced fertilization, elimination of intermediate hosts, and avoiding dense plantings, some organic systems allow the use of copper products. Bordeaux and lime sulfur mixtures stand out as the options with the best results in controlling late blight in this type of production. Bordeaux mixture can be phytotoxic to tomato plants, therefore spraying should start with lower concentrations than recommended (for example: 0,5%) and gradually reach 1,0%. Lime sulfur can also cause phytotoxicity when applied on hot days. Recent research has observed that Bacillus subtillis, Applied preventively, it can reduce the severity of late blight in tomato fields.
Originally from Central and South America, the tomato plant (Solanum lycopersicum) became known after its introduction to Europe by the Spanish in the 16th century. Initially considered poisonous, its incorporation into human food was gradual. Currently, the tomato represents one of the most significant crops on the global agricultural scene. Wide adaptability, high production potential and culinary versatility make this nightshade suitable for consumption "in natura" or industrialized in the most diverse forms. Low calories, considerable amounts of vitamins A, C, B complex, mineral salts and lycopene are characteristics that make tomatoes a food recognized for their antioxidant and anti-cancer properties.
Brazilian tomato farming is widespread throughout the national territory, however, the main production centers are located in the southeast and center-west. Currently, the tomato production chain has great economic and social importance, and assumes well-defined business characteristics, with constant technological advances, land change and advanced management of the production process.
Jesus Töfoli, Ricardo Domingues, Josiane Ferrari, APTA – Instituto Biológico
Article published in issue 79 of Cultivar Hortaliças e Frutas
Receive the latest agriculture news by email
Receive the latest agriculture news by email
