5.6.4S
NEW FUNGICIDE USE ON POTATOES IN BRAZIL

NILCEU RX DE NAZARENO

Instituto Agronomico do Parana, C. Postal: 2301, 80001-970, Curitiba, PR, Brazil

Potatoes are the most important vegetable crop in Brazil. The total potato production is approximately 2.7 million tons/year, with an average yield of 15 tons/ha. Potatoes, produced mainly for the fresh market, are a high-value, cash crop with prices determined by seasonal availability. High yields and attractive tuber appearance needed for sale are achieved through the intensive use of pesticides [1].

Late blight (Phytophthora infestans) and early blight (Alternaria solani) are the most important foliar diseases, with late blight being the more devastating under favorable environmental conditions. In susceptible cultivars without chemical treatment, late blight epidemics start 25-30 days after sprout emergence and reach 100% severity levels 25-30 days thereafter. This can result in total yield loss. Early blight epidemics start later in the season, with less risk of yield losses than late blight.

Nearly 20% of the total cost of potato production involves crop protection chemicals. Of all fungicides sold in Brazil, 13.6% (US$ 25.6 million) were applied to potatoes [2]. In Parana state, with almost 25% of the countryís production, the average number of fungicide sprays per growing season varies from 8.7 to 24.7 depending on the level of technology adopted [1]. Under these conditions, disease epidemics are controlled, but significant chemical loading into the environment is a consequence.

Disease control in potatoes currently is based on a regular calendar spraying system. Fungicide price is an important determinant of the farmerís choice of product. Sub-dosages are not uncommon [1] since farmers, not aware of fungicide resistance risks, try to economize using doses below those recommended. In general, the most widely used protectants are mancozeb and chlorothalonil [1] with applications beginning soon after sprout emergence. Small farmers, who are the majority, continue on a 5-7 day interval, whereas large farmers, not willing to risk their investments to disease, may repeat sprayings on a daily basis during cool, rainy weather. Should favorable conditions persist, farmers start using the more specific metalaxyl- and cymoxanil-based fungicides. Fungicides registered for early blight control are triazoles such as propiconazole, tebuconazole, and difenoconazole as well as protectants, iprodione and procymidone.

Several new fungicides for late and early blight broaden the farmersí options for disease control. In general, these offer a significant reduction in the amount of fungicide applied to the crop, compared to traditional protectants, because of their lower recommended use rates; however, farmers will most likely maintain a weekly or shorter spray schedule due to tradition and fear of control losses. Fluazinam and dimethomorph, recently registered for late blight, have lower application rates than the protectants. Azoxystrobin was recently registered mainly to control early blight, although it also provides some control of late blight. This fungicide has a novel mode of action, offers an alternative active ingredient for disease control, and can result in a significant reduction in the amount of product/ha applied compared to the traditional protectants. Famoxadone, currently in registration, will represent a large reduction in the amount of product/ha applied. It will be available as a pre-mix with cymoxanil for late blight and with mancozeb for early blight. With a better efficiency than mancozeb alone to control early blight, the mixture of famoxadone+mancozeb contains a lower mancozeb concentration, resulting in a 2.4 fold reduction in mancozeb/ha applied compared to the use of mancozeb alone.

Other novel fungicides which are known internationally, such as the systemic acquired resistance type, are not yet available for potato disease control in Brazil.

In conclusion, potato growing in Brazil in the near future will continue to depend on chemical control. Farmers, advisors, and the public look forward to alternatives to traditional fungicides that are safer to use, more environmentally friendly, and yet allow good potato production.

References
1. Nazareno NRX, Brisolla AD, Zandona JC, 1995. IAPAR, Informe da Pesquisa, 114, Londrina, Brazil.
2. Conceicao, MZ, 1996. Curso de Protecao de Plantas. ABEAS, Brasilia, Brazil.