Management of cercosporiosis, the main disease in beet crops

The main disease in beet crops is cercosporiosis, caused by Cercospora beticola Sacc. It is responsible for the total destruction of the leaf blade and, consequently, a reduction in productivity. Its widespread occurrence can represent a reduction in productivity of 15% to 45%. The disease finds favorable conditions when there is high relative humidity (above 90%) and temperatures between 22ºC and 26ºC.^oC. Despite the chemical control available, cercosporiosis normally makes it unfeasible to sell plants in bundles, due to the appearance of diseased and senescent leaves (Tivelli et al, 2011; Puiatti & Finger, 2005).

The characteristic symptoms are observed on older leaves as they are more susceptible (Weiland & Koch, 2004). Initially, these are spots that evolve and tend to reach 4mm to 5mm, with a more or less rounded shape, a clear center and edges with a red-purple perimeter (Figure 1). As the lesions enlarge, they become grayish in color, however, with necrosis, the injured tissue falls and the leaf becomes perforated (Figure 2). The increase in the number of lesions and the growth of the area induce senescence and a significant reduction in the leaf area (Figure 3). The plant replaces leaves from tuber reserves, which can cause loss of yield and this process is repeated several times during the plant's growth cycle (Figure 4).

Epidemiology

The pathogen is considered necrotrophic and can survive in cultural remains, contaminated seeds and/or seedlings, host plants such as chard, Chinese cabbage and sugar beet and remaining plants. The spread is carried out by the action of wind, water droplets and insects, which, when depositing spores on the plant's leaf surface - combined with favorable environmental conditions of high relative air humidity (greater than 90%) and temperature between 22ºC and 26°C -, germinate and emit the germ tube from which they penetrate only through the stomata. After penetrating, the hyphae grow intercellularly in the leaf mesophyll, branching several times. Cellulases and pectinases are involved in the colonization process. Toxins such as cercosporinia and betycholine are produced by the pathogen in order to necrotize the plant tissue in the vicinity of the branched hyphae and obtain its nutrients. This occurs mainly on the abaxial side of the leaf, where it becomes the reproduction site for new conidiophores and conidia. The conidia are once again dispersed by wind and rainwater and/or irrigation to begin new cycles of infection that take around 12 days to present new symptoms (Figure 5).

Disease management

Several control practices can be adopted to managethat of cercosporiosis. One of them is the use of certified seed treated with fungicides. Healthy seedlings should also be used, avoiding cultivation in areas close to where the crop and/or chard is grown.

It is necessary to avoid monoculture of beetroot and/or chard in the area and to rotate crops to reduce inoculum in the area. Fertilization must be carried out according to the needs of the crop, avoiding excess nitrogen fertilization.

It is necessary to follow the recommended spacing for the crop, avoiding dense planting. Irrigation must be carried out according to the needs of the crop.

Whenever possible, cultivate in a drained place, avoiding the accumulation of water in the crop. When using a bed, raise at least ten centimeters to reduce humidity on the soil surface.

It is recommended to cover the soil with plant residues in order to reduce humidity in the lower leaves. Leaves removed at harvest time must be buried in a distant location and not returned to the farm without undergoing thermophilic composting. It is also recommended to remove remaining plants and/or tubers after harvest so that they do not become a source of inoculum.

Folders from some vegetable seed companies describe that the All Green genotype is considered highly resistant, while Itapuã, Stays Green and the Carbenet hybrid are tolerant to the disease. However, Filgueira (2007) describes that the majority of cultivars do not present a high level of resistance.

The use of lush syrup has shown control of the disease with an increase in the quality of tubers (Tivelli et al, 2011). However, the use of chemical control with fungicides has been the main means of combating the disease. Currently, 20 products are registered in the Agrofit System, 2014, of the Ministry of Agriculture, Livestock and Supply (Mapa), such as protectors based on copper hydroxide (3), mancozeb (3), mancozeb + copper hydroxide (1) , systemic ones based on Kasugamycin (1), tebuconazole (8), difeconazole (1) and azoxystrobin (2) which is also available in mixture with difeconazole (1). In Brazil, there is still no report on the pathogen's resistance to fungicides, but resistance to triazoles, mancozeb and strobilurins has already been found in sugar beet in Europe, the United States and Turkey (Colak Tumbek et al, 2011; Hanson et al, 2012; Karaoglanidis et al, 2000; Karaoglanidis & Thanassoulopoulos, 2002; Secor G.A. et al, 2010). This fact reinforces the need to rotate active ingredients to avoid the occurrence of pathogen resistance in the country.

Beet production

The beetroot (Beta vulgaris L.) belongs to the Chenopodiacea family by NRSC/USDA (2014) and to Amarantaceae by APG (2014). This vegetable crop originates from European and North African regions with a temperate climate, and is more likely to be planted in places with a cold climate. It is a typically biennial plant, requiring a period of intense cold to pass to the reproductive stage of the cycle, when the floral tassel is emitted, with seed production (Filgueira, 2007). Furthermore, according to Filgueira (2007), in most of the producing regions in the Southeast and South of Brazil, planting occurs mainly during autumn-winter, especially in those low-altitude regions, while in high-altitude regions, it is possible to plant at throughout the year, including during the summer. Therefore, the most suitable temperatures for the development of this plant are within the range of 10ºC to 20°C (Puiatti & Finger, 2005). Heat is a limiting factor for most cultivars. Thus, when planted under high temperatures and rainfall, premature destruction of the leaves by cercosporiosis occurs and the tubers present poor internal color, with light rings (physiological disorder). In such adverse conditions, the flavor is also affected, becoming less sweet (Ferreira & Tivelli, 1989).

In several countries in Europe, North America and Asia, its cultivation is highly profitable and is used for the production of sugar and fodder (Filgueira, 2007). In Brazil, it is mainly consumed in natura, cooked or in the form of juices, as well as an increase in demand for this vegetable in the canning and baby food industries (Tivelli et al.

National beet production is one of the most significant within the national context of the agricultural vegetable market. Although there is no systematic survey by IBGE, there are around ten thousand hectares of this vegetable in Brazil, produced on more than one hundred thousand properties. The Southeast region is responsible for 45% of national production, which represents around 250 thousand tons/year, generating income for more than 500 thousand people per year (Tivelli et al, 2011). In 2012, data released by the Santa Catarina State Supply Center (Ceasa/SC) showed that more than 4.583 tons of table beet were sold. Santa Catarina produces around 65% of the beetroot it consumes and the rest is imported from other states, with emphasis on Rio Grande do Sul, where 17,5% of the total sold came from (SÍNTESE, 2012).

FIGURE 1 – Initial symptom of cercosporiosis presenting circular spots with a light center and red-purple edges

FIGURE 2 – Symptom of cercosporiosis with and without perforations in the center of the injured tissue

FIGURE 3 – Severe symptom of cercosporiosis with coalescence of spots and leaf senescence

FIGURE 4 – Development of new leaves resulting from leaf senescence caused by the disease

FIGURE 5 – Schematic cycle of beet leaf spot

Check out the article in issue 84 of Cultivar Hortaliças e Frutas magazine.