Symptoms of nematode attack and management in carrots

Carrot crops are affected by different pathogens, which often make production unviable. The phytonematodes responsible for large losses in cultivation stand out, constituting a constant concern on the part of producers and interest for researchers.

Nematodes of the genus Meloidogyne they have a wide range of host plants, including more than two thousand susceptible plant species, including several vegetable crops, such as pumpkin, lettuce, eggplant, carrot, cucumber, tomato, pepper, pepper and others.

Knowledge of the occurrence of these pathogenic agents allows the adoption of joint measures, mainly preventive, helping the producer to reduce damage to the crop. The occurrence of these pathogens is not limited to the main producing states such as Minas Gerais, São Paulo, Paraná, Bahia and Rio Grande do Sul. It also finds favorable conditions for its development in the northern region of Mato Grosso, causing total losses in areas commercially cultivated with carrot.

Damage in carrot

Root-knot nematodes (Meloidogyne spp.) represent one of the main phytosanitary problems in vegetables in the tropics. Loss estimates vary greatly between different crops. In carrots, they can reach 100%, depending on factors such as population density, cultivar susceptibility, nematode species, soil class and environmental conditions, in particular, the combination of susceptible cultivars planted in soils with high population levels of the pathogen. However, even at low infestations, production can be compromised by up to 25%. Losses generally occur due to a reduction in the quantity and quality of the harvested product.

The most significant damage results from attack by root-knot nematodes, generally M. unexplained e M. javanica which have greater territorial distribution in different Brazilian regions, although other species such as M.hapla e M. arenaria are observed in isolated areas of the country. In carrot cultivation in Brazil, species of Meloidogyne most common are M. unexplained e M. javanica.

Epidemiology

Several factors interfere with the reproduction and development of nematodes, such as temperature and soil humidity. Soil temperatures around 15ºC to 30°C are optimal for reproduction. They can become inactive between 5ºC and 15°C and between 30ºC and 40°C. Below or above these limits, temperatures can be lethal, depending on the exposure time. As for soil moisture, normally the optimum condition for the plant is the optimum for the nematode. Dry or water-saturated soils are always unfavorable for the survival of these nematodes. Thus, the main factors that affect the survival and movement of Meloidogyne in soil are soil temperature, humidity and aeration.

An important aspect of the life cycle of these pathogens is that the female can produce an average of 500 eggs to a thousand eggs, generally deposited on the surface of the roots, which can be seen with the naked eye. The nematode life cycle is 21 to 45 days, depending on environmental conditions.

Symptoms

Damage caused by root-knot nematodes (Meloidogyne spp.) are reflected in the aerial part of plants, with reduced development, yellowing and premature fall of leaves, unequal size of plants, with varying degrees of dwarfism, in addition to wilting in the hottest hours of the day, due to reduced absorption and translocation of water and nutrients by the host plant, resulting from reduced root volume and disorganized vascular system, due to the formation of galls.

Typical symptoms of the disease in areas with high infestation, as observed in the region of Alta Floresta, Mato Grosso, were the presence of galls on the roots (popularly known as “popcorn”), deformation, bifurcation and changes in the surface of the roots (becoming the rough ones), reduced development of the aerial part, yellowed leaves, reduced size and deformed roots due to the presence of galls. As the disease progresses, there is a reduction in the weight of the roots, plants with short carrots, often forked, which interferes with the classification commercial of the product.

Control measures

The most efficient methods of controlling nematodes are genetic resistance and crop rotation. Considering that once introduced it is practically impossible to eliminate nematodes from the soil, and as carrots are an annual crop, the damage will be greater the higher the initial population levels in the soil. Therefore, control measures that reduce the initial population or reduce the infective capacity of nematodes should be prioritized:

- Prevention: preventing the spread of these pathogens by using nematode-free planting material, avoiding the movement of people, animals, implements and agricultural equipment from contaminated areas to free areas, constitute the most important measures and the best principle of defense for nematode control ;

- Use resistant or tolerant varieties when available: genetic resistance of plants to nematodes is one of the most efficient and economical methods of preventing losses caused by these pathogens. It is worth mentioning that, in Brazil, the development of cultivars adapted for cultivation, especially heat-tolerant cultivars with resistance to the main foliage diseases and with better root quality, has allowed the expansion of cultivation areas to the Northeast and Center regions -West of the country, with planting and harvesting throughout the year in these regions.

- Elimination of crop residues and host plants: This practice does not always receive the attention it deserves from producers. In the absence of the host plant, nematode survival is affected by climatic conditions, such as temperature and soil moisture. After harvesting, the roots of certain plants remain alive for several weeks, serving as a source of inoculum for nematodes or other soil-borne pathogens.

- Crop rotation: it is one of the most recommended methods for managing nematodes in short-cycle annual or perennial crops. Rotation with non-host plants favors the reduction of the pathogen population. In general, two years of successive rotation with grasses and legumes drastically reduces the nematode population in the soil;

- Fallow: maintaining the area without the cultivation of any plant, with periodic plowing and harrowing, followed by keeping the area clean and free of vegetation, can substantially reduce the population of nematodes in the soil, reaching, in the case of root-knot nematodes, 90% after 3-4 months of fallow;

- Antagonistic plants: They are unfavorable hosts, in which the nematode penetrates, but few are able to develop. It is one of the most studied cultural methods for controlling nematodes. Among the most studied species is the genus tagetes, mucuna e crotalaria. There has been a significant reduction in the population of M. javanica in carrots in Brazil, using Crotalaria spectabilis e tagetes spp., and can also be used as green manure;

- Biological control: represents an important alternative nematode control measure, through the use of various predators and parasites, such as bacteria, fungi, insects and other nematodes;

- Chemical control: nematicides registered in Brazil for carrots are granules from the carbofuran group and must be used appropriately according to the manufacturer's recommendations;

It is important to remember that plants with symptoms of nematode incidence must be sent for analysis in a specialized laboratory, in order to diagnose and adopt appropriate control measures.

Final considerations

It is up to the producer and/or specialized professional to carry out prior research before installing the crop, to choose the cultivar that best adapts to the region and to be informed about the main pathogens occurring in the area. Remembering that seeds of several carrot cultivars are normally found on the market, with each cultivar having its own characteristics in terms of root shape, resistance to diseases and, mainly, in terms of planting time. When it comes to nematodes, consider that the most efficient control methods are genetic resistance and crop rotation. After all, the place for galls is not on the roots.

The carrot

The carrot (Daucus carota L.) is a vegetable crop from the Apiaceae family, from the tuberous root group, cultivated on a large scale with wide culinary versatility, making it one of the most cultivated vegetables in the world. Present in the diet of Brazilians, it stands out among the five main vegetables grown in Brazil in order of economic importance. More than 25 thousand hectares are cultivated annually, resulting in a production of more than 750 thousand tons/year

The commercial part is the taproot, tuberous, fleshy, smooth, straight and without branches, cylindrical in shape and orange in color, due to the high beta-carotene content, being considered the best vegetable source of vitamin A. Rich in carotenoids, potassium and fiber , carrots are an important food. Good root development is achieved when it finds optimal physical conditions in the soil to develop without deformation.

Check out the article in issue 83 of Revista Cultivar Hortaliças e Frutas.