Establishment of tomato planting and health

The experiment was established with the variety SVTE8444 (Seminis^®) in a 1.3 ha field with 90% of the plantation under a plastic macro tunnel. The remaining 10% was kept in the open due to the heterogenous characteristics of the field and the geometry of the land. A fertigation system was used, with drip tape (30 cm between droppers) with an irrigation frequency as required by the crop. The setup took place between July and December of 2020 in the municipal area of Copándaro, Michoacán at an elevation of 1870 masl, a subhumid temperate climate with rains in the summer (Cw)(w), an average annual rainfall of 741-959 mm and an average annual temperature between 16 and 18 °C (^{Hurtado-Rico et al., 2006}). The agricultural staff followed the health measures established by the Secretariat of Health of the Federal Government to prevent the spreading of SARS-CoV-2, such as the use of face masks, a social distance of 1.5m between individuals and frequent hand washing. The main phytosanitary activities included the trimming of axillary buds with scissors 20 days after transplanting; fluopicolide and propamocarb in drench to prevent diseases caused by oomycetes in the soil; gentamicin and oxytetracycline to prevent airborne bacterial diseases; and the application of flupyradifurone, thiamethoxam, cyantraniliprole, abamectin and bifenthrin against the main insect pests, such as whiteflies (Bemisia tabaci, Trialeurodes vaporariorum), leaf miners (Liriomyza sp.) and thrips (Frankiniella occidentalis). Seven days after transplanting, damping off was found, with an incidence of 25%, due to which it became necessary to purchase another 5000 plants from another supplier in the municipal area of Yurécuaro, Michoacán, and Infinito (fluopicolide + propamocarb) (Bayer^®) was applied in drench, reducing the incidence of the disease down to 4%. During the development of the crop, according to the empirical knowledge of the farmer and to the book “Diseases of the tomato”, originally ‘Enfermedades del tomate’ (^{Blancard et al., 2011}), the following diseases were found, with an incidence below 1%: damping off, stem rot, pith necrosis, and a viral disease putative to the Cucumber mosaic virus.

Detection and management of the bacterial canker

Fifty days after transplanting, with constant rainfalls, a relative humidity >90% and a temperature between 20 and 32 ℃, whitish lesions were found on the edges of the leaves with an incidence of 10% (Figure 1A), which evolved into chlorotic lesions (Figure 1B). In plants in the open field, the lesions turned necrotic, with the appearance of leaf blight (Figure 1C). Fruits were also found to have whitish spots with a dark center (Figure 1D). Eighty days after transplanting, the plants in the open field displayed greater damage due to the disease with necrotic lesions on sections of the stem (Figure 1F). The distribution of diseased plants was heterogenous in the crop. The characteristics of the lesions coincided with the description of the bacterial canker in tomato (^{Sen et al., 2015}). Samples of foliar, stem and fruit tissue with symptoms of bacterial canker were collected and taken to the Phytopathology Laboratory of the Guanajuato State Plant Health Committee (Comité Estatal de Sanidad Vegetal del Estado de Guanajuato - CESAVEG) for processing and to determine the causal agent using the ELISA immunology test adapted by the CESAVEG (MA-LDF-#2B; MA-LDF-1V). The results confirmed the causal agent of the disease to be Clavibacter michiganensis subsp. michiganensis (Cmm).

Figure 1 Symptoms of bacterial canker (Clavibacter michiganensis subsp. michiganensis) on a tomato plant, variety SVTE8444 in Copándaro, Michoacán, Mexico: A) whitish lesions on the edge of the leaflets; B) chlorotic lesions on the edges of the leaflets; C) necrotic lesions with the appearance of marginal blight on leaflets; D) whitish spots on fruits; E) production of fruits from healthy and diseased fruits inside the macro tunnel; F) death of plants in the open field. 

For the management of the disease, kasugamycin, oxytetracycline, gentamicin and copper hydroxide-based bactericides were applied every seven days, and up to one week before harvest, in the doses recommended by the manufacturer. In addition, the nbelyax-based product Nanoagro solutions Exodus Max^® was used at 15%, which is recommended for bacterial diseases in tomato plants (^{Nanoagro, 2020}). The furrows with diseased plants were labelled using red stakes to avoid manipulating them, since it is a disease that is mainly transmitted mechanically. Ninety days after the transplant, all plants were partially defoliated to obtain an even maturation and to improve ventilation. This activity was performed using cotton wool gloves soaked with the product Microdyn^®, made of an ionized silver solution and bidistilled water. Following the dose recommended by the manufacturer, the product was applied into plastic containers with 10 L of water, and the gloves were then submerged for 5 min; the agricultural staff submerged the gloves into the Microdyn^® solution every 10 min to avoid the accumulation of culture-forming units and the transmission to healthy plants. Starting on day 95 after transplanting, the harvest began, leaving symptomatic plants until the end. The harvest period lasted until 190 days after transplant.

Productive loss putative to the bacterial canker

The aggressiveness of the disease was less in plants covered by the macro tunnel. During the harvest period, the intensity of the damage to these plants did not increase, maintaining a similar production to healthy plants (Figure 1E); the plants in the open field died 120 days after transplanting (Figure 1F). The average crop yield obtained was 130 t ha^-1, which represents a loss in production of 10% in comparison with the yield obtained in the same season in 2019 (144 t ha^-1).

The variety SVTE8444 is robust and the technical datasheet reports a wide spectrum in tolerance/resistance to fungal, viral pathogens or others caused by nematodes. This study displayed susceptibility to damping off and an apparent tolerance to Cmm under the macro-tunnel, since the genetic effect was complemented with bactericides; however, it was susceptible in plants in the open field. This contrasts with reports of greater damage taking place in covered plants (^{Blancard et al., 2011}). The nbelyax-based bactericide proved to be efficient in reducing the aggressiveness of the disease under the macro tunnel, yet it was not efficient on the plants in the open field. Therefore, further studies are required to determine if the reduction in the aggressiveness is an effect of the nbelyax or of the protective cover of the macro tunnel, which helps avoid the accumulation of rainwater and dew on leaves, both of which are factors that favor the development of bacterial canker (^{Blank et al., 2016}; ^{de León et al., 2011}). On the other hand, the use of cotton wool gloves soaked with Microdyn^® was able to reduce the transmission of the disease during defoliation, since no new diseased plants were registered during the harvest period (October-December). There are no studies on the application of ionized silver solutions for the management of bacterial diseases in plants, therefore experiments must be carried out to determine their efficiency under controlled conditions. The 10% reduction in the productive yield and the incidence of bacterial chanker, with the plant health management described, contrasts with reported losses of over 50% and incidences of 70 to 100% (^{Frías-Pizano et al., 2016}; ^{Lamichhane et al., 2011}). It was not possible to determine the origin of the strain found in the tomato plants. According to the experience of farmers, this is the first time Cmm was found in plantations in Copándaro, although it was previously reported in Zamora, Michoacán (^{Frías-Pizano et al., 2016}). It is possible that some of the plants acquired for replanting were infected. However, this is difficult to determine, since the plants don’t normally present symptoms until the flowering stage and one of their mechanisms of transmission is via the seeds (^{Blancard et al., 2011}).

Effect of COVID-19 on the price of tomato

The COVID-19 pandemic had a direct effect on the price of tomato in national and American markets, where the reduction in the number of consumers in sales points lowered the price by up to 50% in comparison with previous years (SMATTCOM, 2021). The economic recession caused by the reduction in non-essential economic activities in Mexico and the USA produced an oversupply of tomato in the main markets. This led to the reduction in the price of the crop in November and December, when prices tend to be highest. The advancement in vaccination campaigns against SARS-CoV-2, which began in Mexico on December 24th, 2020, along with the gradual reopening of socioeconomic and educational activities, could reactivate the demand for tomato and improve sales prices in the 2021 cycle, benefitting farmers.

Productive perspectives in the light of the bacterial canker

The management of diseases in crops such as tomato, which is susceptible to diverse genera of pathogens, was a challenge when obtaining inputs and products for the preventive management became difficult. The services provided by plant health diagnosis laboratories was valuable to apply management strategies for the relevant diseases. This work reports the presence of Clavibacter michiganensis subsp. michiganensis for the first time in tomato plantations in Copándaro, Michoacán. This crop, in the productive areas of the region, must be produced following under strict guidelines to prevent its spread into pathogen-free fields. These include crop rotation for 5 years after finding the presence of the pathogen; the use of certified seed; the establishment of seedbeds in disease-free areas or nurseries; and the disinfestation of tilling material and work tools.

Perspectives in the light of COVID-19

In general terms, it was an atypical year for vegetable farmers throughout the country due to the uncertainty in the market caused by COVID-19. Farmers related to a primary economic activity kept themselves active by investing and providing jobs to rural communities. In the case of this study in particular, agricultural workers have no stable jobs and have the need to work in agricultural fields whenever they are required. One advantage of the place of study was the low number of infections at a municipal level, with 50 positive cases in the period between July and December 2020, which represented 0.48% of the municipal area’s total population and el 0.16% of the total of people infected in the state of Michoacán (^{SALUD, 2021}). Another measure that helped avoid infections was that workers that traveled to the work area did so individually and they were not allowed into the macro tunnel if they were suspected of having symptoms of COVID-19. Inside the macro tunnel, the workers were given one furrow each, which were separated by 1.8 m, considered safe, according to the social distancing measures implemented by the federal government (^{GOBMX 2021}). Another important aspect was that 3 to 8 workers were normally hired for the different activities of the productive cycle to avoid the overcrowding of staff in the work area. This experience suggests that agricultural activities must continue, with the use of recommended measures, since it is an unconfined productive process, it has a low risk of spreading due to it being mostly carried out in the open and with exposure to sunlight, and being a necessary activity for the economic sustenance of agricultural workers, who work temporarily and have limited access to public health services.