Management of ramularia leaf spot in cotton

Initially considered only an end-of-cycle disease and incapable of causing damage, ramularia spot currently holds the position of main disease in cotton crops in Brazil. As most cultivars currently in use do not have complete resistance, chemical control of the appearance of the first lesions on older leaves is one of the most used strategies against the pathogen.

Cotton ramularia leaf spot was first reported in the USA. In Brazil, until some time ago, it only occurred at the end of the crop cycle without resulting in losses. However, with the increase in the cultivated area and the use of susceptible cultivars, the disease began to appear earlier and is currently considered the main disease in cotton in the Brazilian cerrado.

The disease is caused by the fungus Ramularia areola (Atk.) (synonyms: Ramularia gossypii Speg. Ciferi, Cercosporella gossypii Speg.), anamorphic form of Mycosphaerella areola Ehrlich & Wolf.

Ramularia leaf spot appears on older leaves after the first apples are emitted. In Brazil, however, the first symptoms are noticed between the emergence of the first flower bud and the beginning of flowering. Initial symptoms can be noticed during the pathogen's colonization phase, even before sporulation begins. They are small lesions (3mm to 4mm wide) delimited by ribs. Therefore, with an angular shape, on older leaves. Seen from the upper surface of the leaf, the lesions are light green in color. Sporulation begins on the underside of the leaf, giving the lesions a cottony appearance similar to downy mildew. In conditions of high humidity, intense sporulation of the pathogen also occurs on the upper surface of the leaves, occupying almost the entire leaf blade. The colonized points necrotize after the pathogen sporulation period, and in some cotton genotypes, necrosis may occur at the beginning of colonization by the pathogen.

In cases of high disease severity, partial or total defoliation of plants may occur. In these cases, the loss of leaves during the apple formation phase compromises production in the upper third of the plant, reduces productivity in the reproductive structures already formed and induces early boll opening, resulting in loss of fiber quality.

R. areola it survives on lesions in crop residues and the spores produced under these conditions constitute the primary inoculum. It is common for the fungus to survive on native perennial cotton plants. The pathogen is spread by wind, rain or irrigation water, people and machines. Fungal conidia germinate in free water at temperatures ranging from 16ºC to 34^oC, with an optimum temperature between 25ºC and 30^oC. Although free water is necessary for conidia germination, penetration, via stomata, is greater in cycles of wet nights and dry days than in cycles of continuous humidity. Some infections occur after two cycles of wet nights with maximum infection after four cycles.

Less dense plantings and carried out to avoid excessive shading between plants and chemical control are the main tactics used to manage the disease. Most cotton cultivars currently explored in Brazil are, at some level, susceptible to ramularia leaf spot. The use of cultivars with some level of resistance, especially those with canopy architecture, which allows or facilitates aeration, combined with greater spacing and lower plant density, can reduce the severity of the disease. The resistant cultivar BRS 372 was recently launched. Other cultivars with high tolerance are also available on the market, such as BRS 371 RF, TMG 41 WS, TMG 42 WS and TMG 43 WS.

As most cultivars currently in use in Brazil do not have complete resistance to the disease, chemical control is a tactic commonly used to contain the disease, and should be started as soon as the first lesions are identified on older leaves. Constant monitoring of the crop is crucial as the first lesions are difficult to identify before sporulation occurs.

The first symptoms of the disease appear concomitantly with the beginning of the plant's reproductive phase, generally between the appearance of the first floral bud and the opening of the first flower. The damage caused by the disease extends until the end of the crop cycle, being most significant between the beginning of flowering and the opening of the first bolls. After starting to open capsules, chemical control does not bring benefits. In the chemical control of the disease, it is important to know the mode of action and the type of translocation of the fungicide in the plant, to decide which product should be used and when it should be applied. In addition to this knowledge, the alternating use of fungicides with different active ingredients is fundamental, as it is an effective strategy to avoid an increase in the frequency of resistant isolates within the population of R. areola.

At the onset of the first symptoms of ramularia leaf spot, the initial inoculum of R. areola it is low and comes from air currents, cotton leaves from the previous harvest or the first lesions installed on older leaves. At this stage, fungicides from the strobilurin group can be used alone or in a pre-made mixture with other fungicides, as they are very effective in preventing the germination of spores and also have an eradicating effect. Delays in starting the first application reduce control efficiency and may even be economically unfeasible. Before the end of the residual period of the fungicide, the plants must be monitored again, because, if new lesions with sporulation (whitish lesions) are found in the middle third of the plant, it is recommended to start the second application, preferably with a fungicide belonging to a different chemical group than that used in the first application. This strategy prevents the addition of inoculum in the area and protects plants during critical periods in need of photoassimilates, in addition to reducing the risk of the emergence of fungicide-resistant isolates of the pathogen. Recently launched molecules should also be used in product rotation, such as prothioconazole, cresoxim-methyl and fluoxastrobin, in addition to fungicides called SDHI (succinate dehydrogenase inhibitors) from the chemical family of pyrazoles-carboximides such as boscalid, fluopyram and fluxapyroxade, as they were little exposed to the pathogen population. Other fungicides, such as Mancozeb, are also important in the chemical control of ramularia leaf spot and should be used in rotation with strobilurins and triazoles.

Depending on the cultivar in use, a third application of fungicides is not necessary, especially for those with some level of resistance, early cycle and cultivated in the second harvest. For susceptible plants, with a medium to late cycle, grown in a single crop, other applications may be necessary. Subsequent applications depend on the progress of the disease after 80-90 days of emergence. Active principles that are part of the registration list against the disease can be seen in Table 1. The commercial names of products against ramularia spot are listed on the Agrofit System website (http://agrofit.agricultura.gov...).

 The Embrapa Algodão breeding program has selected genotypes resistant to ramularia spot. The selection of resistant lines has been possible even in populations originating from parents with only partial resistance, demonstrating that there is transgressive segregation, therefore polygenic inheritance for the character. The crossing between the cultivars Cacique and Ita 90 generated a population in which several resistant lines were selected: CNPA GO 2007-419, CNPA GO 2007-423, CNPA GO 2008-1265, CNPA GO 2008-1266, CNPA GO 2008-1271 and CNPA GO 2009-204. Another example of transgressive segregation is the resistant lineage CNPA BA 2003-2059, which was selected from a population involving susceptible and partially resistant parents [(Stoneville 373 x BRS Itaúba) x Delta Opal].

The CNPA GO 2007-423 line was released as a cultivar, called BRS 372, with total resistance to disease and high fiber productivity. Another recently released resistant cultivar is BRS 371 RF, [(CS 50 x BRS Facual) x Suregrow 125 RF], which also has transgenic resistance to the herbicide glyphosate. Currently, efforts are being made to obtain molecular markers linked to the ramularia leaf spot resistance genes that could accelerate the process of developing resistant cultivars.

Cotton and diseases in Brazil

Brazil is currently the fifth largest cotton producer in the world. The vast majority of this production is carried out in Brazil's cerrado. The high fiber yields achieved in this ecosystem, due to genetic advances and improvements in production systems, make the crop economically viable.

The current production system, based on large areas and few cultivars, many of them susceptible to more than one disease, leads to the worsening of diseases previously considered to be of little importance; it also enables epidemic outbreaks of new diseases.

Nelson Dias Suassuna and Wirton Macedo Coutinho, Embrapa Algodão

Article published in issue 206 of Cultivar Grandes Culturas.