Leaf spots: permanent threat to cotton crops

The cotton crop has been affected by a significant number of diseases since it migrated to the Brazilian cerrado, making the region the largest producer of cotton in Brazil and the country the fourth largest producer and second largest exporter of the fiber in the world. Although soil-dwelling pathogens represent a permanent threat, it is leaf spots that have caused the greatest damage.

In the early years, ramulosis predominated, caused by Colletotrichum gossypii var. cephalosporioides as the disease with the most damage to the crop. However, resistant cultivars and the earlier emergence of ramularia spot led producers to apply fungicides in the early stages of development to stop the spread of the disease. The combination of more resistant cultivars and early application of fungicides made ramulosis practically disappear from the Brazilian cotton farming scene.

Other leaf spots have caused concern to producers over the years, such as Alternaria and Stenphylium spots (Alternaria alternata and Stenphylium solani), myrothecium (Mirothecium roridum) and angular spot, caused by Xanthomonas citri subsp. Malvacearum.

Among the diseases that have threatened the cotton plant, the one that is here to stay and today represents the greatest threat to its yield is, without a doubt, ramularia spot. The disease is considered the main disease affecting cotton in Brazil. Its occurrence is observed in all producing regions of the cerrado, where more than 90% of Brazilian cotton is produced. It can also be found in colder and more humid regions of the semi-arid region, as well as in São Paulo and Paraná, to a lesser extent. As the disease spreads upwards in the plant, it is common for leaves to fall in the lower third, when control measures are not adopted at the appropriate time, or when chemical control does not act effectively, causing significant losses. Intense defoliation can also cause the bolls to open prematurely, damaging fiber quality.

The damage caused to the cotton crop is significant and the economic losses are of great magnitude. Studies carried out by Araújo, et al., (2020) demonstrated that ramularia spot caused losses of approximately US$360 million in the 2019/2020 harvest, considering crop losses and the cost of chemical control. It must be taken into account that the best fungicide applied today to cotton crops to control the disease achieves, on average, a control level of 84%.

Ramularia leaf spot control is based on the use of resistant cultivars and chemical control. Considering that the majority of cultivars in use in Brazil do not have complete resistance to the disease, the latter is an indispensable tool for its management. Fungicide applications should be started as soon as the first signs of pathogen presence or the first lesions are identified on older leaves. Crop monitoring must begin early, as injuries are common before 40 days after emergence.

For chemical control to present a satisfactory response, between four and eight sprays must be made, with up to 12 applications being required during the crop cycle (LAMAS and CHITARRA, 2014). In this sense, the choice of the product, knowledge of its mode of action, the time and interval of application, as well as the alternating use of active ingredients with different modes of action, represent fundamental aspects for the efficiency of chemical control, in addition to being an important strategy to reduce the risks of the emergence of resistant populations of the pathogen (DIAS, 2017).

Recent studies on the efficiency of fungicides to control the disease have pointed to a reduction in the efficiency of some Triazole fungicides and, at the same time, identified fungicides with high performance and with different modes of action that are already on the market and others in the development phase. Temporary Special Registration at the Ministry of Agriculture. Multisite fungicides based on copper and chlorothalonil showed a synergistic effect when associated with site-specific fungicides and may represent an alternative for managing pathogen resistance to active ingredients (ARAÚJO et. Al. 2020).

The cotton production system in the cerrado is associated with the planting of soybeans and corn, either in rotation or in succession. This system has been repeated over the years and the succession of these crops may be the cause of the adaptation of some pathogens from one crop to another. In Bahia and Minas Gerais, the presence of white mold caused by Sclerotinia sclerotiorum in cotton has been detected for some time (CHARCHAR, 1996). The occurrence occurred in cotton plantations under central pivot previously cultivated with beans. The disease was spread to almost the entire Brazilian cerrado, although epidemic outbreaks have always been occasional. Planting sunn hemp as a cover crop is a highly relevant factor for the multiplication of sclerotia of the pathogen given its susceptibility to S. sclerotiorum.

Another disease that has become a threat to cotton crops is target spot (Corynespora cassiicola), previously a problem restricted to soybeans. Most of the cotton planted in the cerrado occupies the recently harvested soybean areas. The pathogen was reported for the first time in Brazil, in Mato Grosso, in 1995. The disease has been causing damage due to its prevalence throughout the harvest and its significant damage. There are still no studies that determine the possible losses caused, but the disease is aggressive, since the necrotic lesions can reach a diameter of up to 20 mm and the leaf fall is intense and premature.

It has already been demonstrated that C. cassiicola isolates from both cotton and soybeans can cause target spot in both hosts (GALBIERI 2014). In this sense, the production of fungus inoculum in soybean crops can constitute a source of primary inoculum in the subsequent cultivation of cotton in the second harvest or even in the subsequent harvest in single-crop cotton (SUASSUNA et al, 2015).

More recently, the fungus Colletotrichum truncatum, a pathogen historically associated with soybean cultivation, was observed affecting cotton crops in the Araguaia River Valley region, precisely in the municipalities of Canarana and Bom Jesus, in Mato Grosso (JULIATTI et al., 2020). The extent of the damage and losses caused by this new disease is not yet known, but the fact is that it is necessary to maintain its monitoring to avoid surprises in the near future. As we can see, there are adjustments that need to be made in the crop succession system involving corn, soybean and cotton crops, so that the emergence of new phytosanitary problems cannot cause greater damage to cotton producers in the cerrado.

José Otávio Menten, president of the Sustainable Agro Scientific Council; Alderi Emídio de Araújo, Researcher and General Head of Embrapa Algodão