Prevention and management of castor bean diseases in the Federal District

The castor bean (

Ricinus communis

) is an oilseed plant of tropical origin, possibly from Ethiopia, east Africa. It adapts well to regions with a hot climate, requiring 120 to 160 days for complete development and fruit production. With moderate to high resistance to drought, it needs 500 mm of precipitation for satisfactory growth, development and production. The ecological conditions necessary to express its productive potential also include air temperature between 20 and 30ºC, altitude varying from 300 to 1500 meters.

The plant is adapted to different types of soils, except very clayey ones subject to waterlogging, saline and/or sodium, with a high exchangeable sodium content. According to the agricultural climatic risk zoning for castor bean cultivation in the Federal District, prepared by the Ministry of Agriculture, the recommended time for sowing castor bean in the DF comprises the period between the first week of October and the last week of December.

According to the rainfall history recorded in the DF, from November to March, the monthly average is 200 mm, with around 19 days with rain per month. This humid environment, with continuous or alternating periods of rainy days, and mild temperatures around 25°C, are ideal conditions for infection, multiplication and dissemination of most plant diseases, including those that affect castor beans.

Therefore, despite favoring the development of the crop, these conditions also favor the development of fungal diseases – such as castor bean gray mold, cercospora spot, alternaria spot and seedling damping-off – and bacterial diseases, such as bacterial leaf spot. Even though it is considered a rustic plant and well adapted to the Brazilian soil and climate, the castor bean plant is greatly affected by these pathogens, which can cause significant economic losses.

Therefore, for there to be adequate management of crop health, it is necessary to plan planting at a time favorable to the plant's development. However, it should be noted which periods are when the crop is most susceptible to diseases, so that this period can be avoided. Delaying or advancing planting, within the period recommended for the crop, can be an alternative to prevent plants from being exposed to long periods of constant rain during the reproductive period.

Another alternative to avoid the emergence of large epidemics is to reduce sprinkler irrigation during flowering or carry it out during the night, coinciding with the period when dew forms on the plants. Crop rotation, another management tool, is recommended for any annual species and, as in other crops, it is necessary that the cycles are interspersed with those of non-host species of the main castor bean diseases to promote the reduction of the pathogen inoculum already present in the area.

Another management technique is the elimination of crop residues by incorporating straw into the soil through plowing or harrowing. Weedy plants present on roadsides, ravines and riverbanks must also be eliminated. You can also choose areas that were not cultivated with castor beans in the previous period. Finally, when the disease takes hold and other forms of management do not contain the spread of the epidemic, it is necessary to resort to the use of chemical pesticides.

resistant cultivars

Although the alternatives presented provide results, the best management strategy that the producer can use is the use of improved cultivars, especially if they present levels of resistance to the main diseases, in addition to providing high productivity. Currently research institutions, such as Embrapa, have castor bean cultivars and hybrids with productivity ranging from one thousand to four thousand kilograms per hectare and which are recommended for different regions of Brazil.

Among these varieties, the ones that have an open bunch architecture, with the berries spaced apart, stand out as tolerant to castor bean mold, ensuring good aeration and rapid evaporation of rainwater/irrigation water or dew that may settle. among the berries. This characteristic prevents the bunch from staying wet for a long time, which would create a humid microclimate favorable to infection and sporulation of the fungus. Plants with open branching and emission of clusters above the crown and leaves have greater ventilation and reduced humidity inside the crown, which makes it difficult to establish most aerial part diseases, such as leaf spots, in addition, the spacing between rows and between plants must be managed to provide good aeration between plants, without a significant reduction in the stand.

tipping

Seedling tipping over, another important phytosanitary problem in crop implementation, is related to soil fungi that promote stand reduction. Among the fungi that cause seedling tipping, the most common are Rhizoctonia, solani, Fusarium sp., Scerotium rolfsii and Alternaria sp. The problem can be avoided by choosing an area for planting without a history of the occurrence of these pathogens, areas with well-drained soils, and shallower sowing in case of precipitation forecast in the days following planting.

Another alternative is to use certified seeds, which guarantees their health and prevents the installation of new pathogens in an area previously without the pathogen. Seed treatment with fungicides and insecticides is also a management practice for soil pests. However, it is important to highlight that there is still no registration with the Ministry of Agriculture of pesticides for this purpose in castor beans. This disqualifies its recommendation, despite research showing promising results.

Angelo Aparecido Barbosa Sussel

PhD in Phytopathology and researcher at Embrapa Cerrados.

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