Importance of care from pre-planting to post-harvest with potato y virus in tomatoes

In recent years there have been increasing reports of a very severe disease in tomato plants. Infected plants show necrosis (burning) of leaves and, in extreme cases, crops have been completely destroyed, causing enormous damage to producers. The disease is known in some regions as Mexican fire. It is not known where this name came from. This virus has already been reported in the main tomato producing states and occurs with relevant frequency in the Federal District, Goiás, Minas Gerais, São Paulo and Rio Grande do Sul.

The disease is caused by potato virus Y, or potato virus Y (PVY). This is a potyvirus, which has an elongated and flexible particle and is made up of RNA and a protein coat. The virus is transmitted in nature by several aphid species in a non-persistent and stylet form. When making a test bite on an infected plant, the insect immediately acquires the virus. When leaving the infected plant and visiting a healthy one, at the time of the test bite it will already be able to transmit the virus. Therefore, chemical control is not very efficient in controlling the disease.

Sometimes the insecticide can cause excitement in the insect, causing it to move in a disorderly manner and carry out numerous test bites, which even tends to increase the number of infected plants. The best option for chemical control is to carry it out on the crops where the disease is present, reducing the risk of being the source of viruses and aphids for younger crops. Remembering that you should not plant crops in a staggered manner, with new and old plants next to each other, as pests and pathogens will quickly move from the old plantation to the new one.

The virus can infect tomato plants, pepper plants, potato plants, tobacco plants and countless others, including weeds. Therefore, it is recommended to keep the crop free of weeds, inside and outside the plantation. It is also very important to eliminate aphid host plants, as they can easily acquire and transmit the virus. It is recommended to install wind barriers, which make it difficult for winged aphids to move from one crop to another. In general, winged aphids are more efficient in transmitting the virus, due to the ease of movement between plants in the crop and also to other crops. Infestation of wingless aphids in tomato crops is rare.

Symptoms

Symptoms of Mexican fire disease begin with the appearance of necrotic spots on plant leaves, evolving into rounded or irregular spots and can cover much or all of the leaf. They generally appear in developed plants, in the middle part of the plant. As the disease progresses, the burning symptom is clearly seen from the bottom up on the tomato plant, reaching the top of the plant. In general, these spots are more quickly and easily seen on the underside of the leaf, gradually becoming visible on the top side of the leaf.

It is common for symptoms not to be seen on older leaves, most likely because plants in the initial phase are more protected by the grower to avoid visits from aphids and other insects. In infected plants, there is a drastic reduction in the number of fruits, but no symptoms are observed in the fruits. The symptoms caused by this virus can evolve, causing dwarfism throughout the plant. Depending on the season, infected plants may be more chlorotic than healthy ones. Complete burning of a large part of the plant is seen in cases of early infections. Sick plants are more common at the edges of the crop and especially close to aphid and virus hosts.

PVY was an important disease for tomato, a disease known as "pinheirinho", but the incidence was reduced after the work of breeder Hiroshi Nagai, from the Agronomic Institute of Campinas, who launched cultivars with resistance to PVY infection, for example Ângela Hyper. Currently, as it is no longer considered an important disease, there is no mention of resistance to PVY in the brochures of commercial tomato cultivars. For this reason, it is not known whether the materials are resistant to PVY infection. As the disease becomes more and more important, seed marketing companies should evaluate and include this information in brochures advertising commercial hybrids.

Virus management recommendations

Before planting

It is essential that planning is carried out before planting tomato plants and, whenever possible, talking to producers in neighboring areas, to find out which insects and diseases occur frequently. In this way, control can be anticipated, avoiding large infestations of pests, such as aphids, in addition to reducing the incidence of various diseases. Regional planning, selecting the best times and locations and establishing an agricultural zoning focused on tomato cultivation to reduce the occurrence of pests, is the recommended measure and needs to be exercised by producers. Discussion between producers and the creation of a producer association can help a lot in defining measures together, beneficial to all producers.

Practices such as soil preparation with deep tillage, balanced fertilization tailored to the type and condition of each soil and adequate irrigation can contribute to promoting increased plant resistance to various diseases. Never plant before eliminating crop residues from previous crops. It is preferable to wait a minimum period of one month between the destruction of crop residues and new planting, to increase the efficiency of eliminating pests and pathogens present in the area.

It is very important to know the most important diseases in the region and the susceptibility of the selected materials to these diseases, always prioritizing cultivars that are more resistant to key pathogens. Tomato seeds are, in general, very expensive. Always use healthy and vigorous seedlings, produced under screens with anti-aphid screen, and with the application of protective systemic insecticides. If the seedlings are prepared in nurseries, inspect the facilities and monitor the procedures practiced by nurseries. Nurseries with anti-chambers help reduce the chances of insects entering. There are excellent commercial nurseries, producing high quality seedlings.

 Whenever possible, plant in places with no history of PVY occurrence, whether in tomato or other crops. When planning planting, study the main wind direction to avoid installing the crop in a location that receives wind from areas with tomato plantations or other plants susceptible to PVY.

It is recommended to plant away from tomato, pepper and potato crops, always analyzing the agricultural landscape before planting. Building natural or artificial barriers between crops reduces the movement of insects and can contribute to reducing the incidence of diseases with airborne vectors, such as Mexican fire. Elimination of weeds that host PVY or aphids must be carried out. Ideally, planting should be carried out inside greenhouses with anti-aphid screens to prevent the entry of aphids.

During planting

The elimination of weeds in and around the crop must be carried out frequently, preventing the multiplication of aphid hosts and various viruses. Chemical, biological or mechanical control against aphids must be carried out frequently, both on tomato plants and on other plants in the area. If necessary, control aphids outside the crop. Pruning tools, as well as other utensils, must be carefully sanitized after use on each plant, as PVY can be mechanically transmitted by the utensil.

The removal and elimination of plants with symptoms of Mexican fire must be frequent at the beginning of the crop, avoiding an increase in the incidence and infection of new plants.

After harvest

The ideal is to apply insecticide to eliminate aphids before destroying crop residues. The destruction of crop residues must be done immediately after harvesting, before planting new seedlings. The successor crop must be a plant that is not susceptible to PVY.

All management recommendations listed will not be efficient if not carried out in a strategic and integrated manner with producers in the region. To control the virus that causes Mexican fire, aphids and weeds, all management measures must be carried out in a coordinated manner so that the plants can express their full productive potential and produce fruits of high quality, flavor, size and appearance. 

Vivian dos Santos Lucena Leandro, José Luiz Pereira, Moana Lima Tavares-Esashika and Alice Kazuko Inoue-Nagata, Embrapa Hortaliça

Growing Vegetables and Fruits December/January 2021

With each new edition, Cultivar Hortaliças e Frutas publishes a series of technical content produced by renowned researchers from all over Brazil, which address the main difficulties and challenges encountered in the field by rural producers. Through research focused on controlling the main pests and diseases in vegetable and fruit cultivation, the Magazine helps farmers in the search for management solutions that increase their profitability. 

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