The survey conducted in São Gotardo made it possible to ascertain the general status of garlic crops in the Alto do Paranaíba region. It was possible to observe that even the four crops considered practically “virus-free” actually presented 2.9 to 11.9% symptomatic plants in the visual inspection. However, when plant samples were analyzed in the laboratory, the incidence was even higher, indicating that newly infected plants may not show symptoms but test positive when analyzed by RT‒PCR. As the symptoms in these three crops with the lowest incidence were milder, they could have gone unnoticed by farmers, who do not always have the necessary training to recognize them.
Carlaviruses are known to be present in garlic crops worldwide, and infection by species of this genus alone does not cause severe symptoms in infected plants (BARG et al., 1993; BELLARDI et al., 1994). However, the occurrence of mixed infections of carlavirus with potyvirus can lead to more severe symptoms (TAKAICHI et al., 1998), as observed in areas G, H, I and J.
Viral diseases are known to cause degeneration in vegetative propagation materials due to the accumulation of the pathogen in successive cultures of the same clone (MELO FILHO et al., 2006; THOMAS-SHARMA et al., 2016; BESSA et al., 2021). As a result, the pathogen becomes the primary agent of significant crop losses. The genera Allexivirus, Carlavirus and Potyvirus cause the degeneration of cloves used in garlic propagation, leading to reductions in vegetative vigor, yield potential and quality of the bulbs across generations (MELO FILHO et al., 2006; RESENDE et al. al., 2006; RESENDE et al., 2016; BESSA et al., 2021). It is common for garlic farmers to use their own planting material as seed, which combined with inadequate storage exposes the crop to a high rate of viral infection in subsequent plantings. Mixed infection with species of the genera Carlavirus, Allexivirus and Potyvirus can cause more severe symptoms and consequently higher yield reduction (TAKAICHI et al., 1998; LUNELLO et al., 2007), as observed in six of the 10 studied areas.
Lunello et al. (2007) compared “virus-free” plants with those infected by LYSV and by a viral complex (Potyvirus, Carlavirus and Allexivirus). Infection with LYSV alone led to a 28% reduction in bulb weight, whereas mixed infections led to a 74% reduction.
The greatest damage to garlic production has been attributed to infections caused by OYDV and LYSV, the two potyviruses already reported in the country. These viruses alone can cause a reduction in bulb mass of between 39% and 60% and between 17% and 54%, respectively, depending on the cultivar (MOURA et al., 2013). The presence of these species in all the areas visited in the present study provided a better assessment of the symptoms and raised the alert for the need to replace the seed bulbils for the next year's plantings. The milder symptoms observed in areas A, B and C were probably due to recent infections during the current season and did not have a major impact on production. However, if the bulbils produced are replanted, they will likely give rise to plants with stronger symptoms and lower productivity.
Infected seed bulbils can be used for planting up to a certain number of remultiplications without causing significant losses to the producer. Silva et al. (2010) analyzed virus-free and infected seminoble cultivars during nine years of cultivation and found a reduction in the productivity of these materials over successive cultivation cycles. Mituti et al. (2015) analyzed the reinfection rate of the viral complex and confirmed that the use of seeds bulbils after three consecutive years led to a production level similar to that of garlic seeds with 100% infection.
The elimination of viruses is performed from the meristem culture, providing an increase in plant vigor and greater productivity and quality of the final product. However, the nutritional requirements of garlic derived from tissue culture are still poorly understood, and many studies will be needed to make the best use of its high yield potential (RESENDE et al., 2016). Thus, mineral nutrition is not always performed properly, which causes losses not only in production but also in the postharvest conservation of the bulbs (MACÊDO et al., 2009).
As the management practices adopted vary with the area and with the technical preferences of the producer, the productivity of plants with different infection rates can also vary. In this study, it was observed that among the areas that presented 100% symptomatic plants, the severity of symptoms was much higher. This may be related not only to a higher incidence of virus in the plants but also to the concentration of virus in the cloves used in planting due to the number of remultiplications in the field.
The regular spatial pattern observed was probably due to the healthiness of the seed cloves used for planting, even in crops with low incidence. Gitaitis et al. (1998) observed that Orthotospovirus tomatomaculae (TSWV) in pepper and tomato showed an aggregated spatial pattern, indicating that most infections emerged from primary transmission. A study by Della Vecchia et al. (2007) also showed an aggregated spatial pattern for Tomato yellow vein streak virus, Begomovirus, which was attributed to the concentration of symptomatic plants at the edges of the greenhouses due to the arrival of insect vectors from areas outside the greenhouse.
Likewise, the random distribution patterns observed in areas with lower virus incidence must have resulted from the use of seeds of superior quality, such as those subjected to clonal cleaning. Thus, the spread of the viruses must have occurred mainly through aphid vectors, which must have acquired the virus in neighboring areas containing older and contaminated plantations. On the other hand, clonal cleaning does not always eliminate 100% of viruses and does not prevent natural reinfection with viruses. Therefore, efficient detection of viruses and disposal of infected plants for seed production are fundamental practices for preserving the production potential of plants (FERNANDES et al., 2013). In particular, a single infected garlic plant can produce an average of 12 cloves, significantly multiplying the infection rate in the next generation.
On the other hand, the fact that 40% of the crops showed 100% viral infection highlights the importance of using virus-free garlic seeds to obtain more vigorous plants with better bulb physiological quality, resulting in increased productivity in the following plantings (RESENDE et al., 2016). Therefore, the management of garlic viruses in the São Gotardo region is currently not efficient. The scarce supply of seeds of high phytosanitary quality and the lack of knowledge of producers about the true cause of the symptoms shown by the plants, together with the lack of knowledge of the management of viral diseases, seem to be the preponderant factors for the current scenario observed for garlic in this region.