How to control target spot on cucumber

Target spot (Corynespora cassiicola) is one of the main diseases that affect cucumber crops, with the capacity to reduce productivity by up to 60%. To carry out control, it is necessary to pay attention to aspects such as choice of hybrid, plant density, irrigation system and management, environmental management, nutritional status of plants and cultural practices.

Causal agent of ‘target spot’ or ‘corynespora spot’, Corynespora cassiicola ((Berk. & Curtis) Wei, 1950) has been reported in more than 360 host species around the world. In Brazil, within the group of vegetables and fruits, this pathogen has caused significant damage to cucumber crops (Cucumis sativus L.) with a reduction of up to 60% in productivity (Verzignassi et al., 2003). The high losses in the parthenocarpic cucumber crop are related to the low efficiency of the fungicides used to control the disease, however, practices such as choice of hybrid (different degrees of resistance), plant density, irrigation system and management, environment, nutritional status of plants, cultural practices, among others, can be related to the success or failure in controlling the disease in the field.

Leaf lesions begin as small chlorotic spots, called ‘Flecks', which evolve into brown to dark brown spots, with a light center, circular to irregular in shape, with the absence or presence of halos (chlorotic, reddish-brown or dark brown) and the presence of areas with a waterlogged appearance, which may cover 1,0mm to 20,0mm in diameter. As the disease progresses, the lesions coalesce, with extensive necrotic areas and tissue and plant death observed. The fruits are not directly affected by the disease, however, development problems, such as deformed and reduced-sized fruits, can be seen in severe attacks.

The survival of the pathogen can occur in crop residues for up to two years, in saprophytic form or through specialized resistance structures called chlamydospores (Oliveira et al.; 2012). In addition to cultural remains, alternative hosts, such as weeds (trapoeraba, assa-peixe and lantana) and commercial crops (soybean, tomato, acerola, papaya, among others), when bordering areas where this cucurbit grows, can contribute as an initial inoculum. for the onset of the disease in the field.

In work carried out at the Phytopathology Laboratory of the State University of Maringá (UEM), Vida et al. (2004) (Personal communication), through the filter paper test and the sand transmission test, reported the presence of the pathogen in seeds and transmission to seedlings of four parthenocarpic cucumber hybrids ('Natsubayashi', 'Hokushin', 'Tsuyataro' and 'Samurai'). According to the authors, the contamination level reached 35% in ‘Tsuyataro’ seeds. C. cassiicola It is associated internally and externally with cucumber seeds. Therefore, seed transmission must be considered when aiming to control the disease.

Regarding the management of the planting environment, the crop must be positioned in a way that favors air flow inside, optimizing the removal of excess moisture and consequently preventing the disease. Another way to reduce humidity is related to the spacing between rows between plants, and the conduction of the main stem and lateral stems. Dense cultivation favors an increase in relative humidity and supply of free water on the leaf surface, factors that contribute to the occurrence of the disease in the field.

When fertilizing, excess nitrogen fertilizers favor the development of softer tissues and an increase in the crop cycle, which can predispose the crop to attack by C. cassiicola. Furthermore, excess of this element will result in extra growth of aerial organs, stem, lateral branches, petioles and leaves, which will promote less air circulation inside the crop, increase in relative humidity and temperature and, consequently, increase in severity. of the disease.

Controlling this disease in the field begins with environmental management. The disease is favored by high temperatures and high relative humidity, the presence of free water on the leaf surface caused by long periods of rain or inadequate irrigation conditions, associated with poor ventilation/aeration conditions in the cultivation area. The spread of the pathogen, after the start of the epidemic, occurs through the dispersion of spores (conidia) produced at sites of infection through wind and splashing water. This disease tends to be more severe in protected cultivation areas (greenhouses).

In the field, the choice of hybrid/variety to be cultivated, in areas where the disease is recurrent, can represent an important tool for control, since they present different degrees of resistance/susceptibility. Oliveira et al. (2006) and Terramoto et al. (2011) identified among parthenocarpic cucumber hybrids, that the hybrid 'Tsuyataro' showed greater susceptibility, while 'Natsubayashi', 'Taisho', 'Nikkey' and 'Yoshinari' showed lower susceptibility to C. cassiicola.

One way of preventing infections in the first stages of crop development is the treatment of seeds (since the pathogen survives and is transported by seeds) as well as the treatment of seedlings still in trays, with systemic fungicides, which contributes to that the seed-borne inoculum potential is satisfactorily reduced, thus contributing to stand uniformity and delaying the onset of disease epidemic cycles.

Cultural remains constitute another form of survival for the pathogen, and symptomatic plant tissue is a source of secondary inoculum for the disease. The destruction of plant remains at the end of the crop cycle, as well as the removal of symptomatic senescent plant tissue during the crop cycle, contribute significantly to the reduction of the initial inoculum in the following crop cycle and secondary cycles of the disease respectively. 

Regarding the chemical control of Target Spot on parthenocarpic cucumber, recent research results indicated the low efficiency of fungicides in the curative treatment of this disease. Significant results have been observed in protective treatments. Oliveira (2008), evaluating the efficiency of different groups of fungicides to control the disease in the Tsuyataro hybrid, observed that the mixture pyraclostrobin + epoxiconazole (not registered with the Ministry of Agriculture, Livestock and Supply for the crop) and azoxystrobin promoted greater disease reduction when applied preventively. Similarly, Teramoto et al. (2011) identified azoxystrobin as the most efficient treatment for controlling the disease in Tsuyataro and Nikkey. Both authors reported the low efficiency of thiophanate-methyl in controlling the disease, suggesting resistance of the pathogen to the active ingredient, as verified in studies carried out in Japan in 2004.  

Ricardo Oliveira, João Vida, Dauri Tessmann, UEM; Jefferson Fernandes do Nascimento, UFRR; Paulo F. Maraus and José U. T. Brandão Filho, UEM

Article published in issue 80 of Cultivar Hortaliças e Frutas