Sensitivity of Brazilian Isolates of Phytophthora infestans to Commonly Used Fungicides in Tomato and Potato Crops

Abstract

Isolates of the US-1 and BR-1 clonal lineages of Phytophthora infestans, collected from tomato and potato fields of two main producing regions of Brazil (south and southeast), were tested for sensitivity to the systemic fungicide metalaxyl, plus the three protectant fungicides mancozeb, chlorothalonil, and cymoxanil. For metalaxyl, one agar test and two leaf-disc tests were carried out. For all tests, metalaxyl insensitive (I), intermediately insensitive (II), and sensitive (S) isolates were detected. There was no association among metalaxyl sensitivity and region, lineage, or host. In the agar test, 35.0% of 210 isolates were classified as I, 36.0% as II, and 29.0% as S. US-1 isolates were classified as 36.1% I, 30.6% II, and 33.3% S. BR-1 isolates were 33.3% I, 23.8% II, and 42.9% S. In leaf-disc test 1 (240 isolates tested), 24.3% were classified as I, 35.0% as II, and 40.7% as S. Isolates of US-1 were 21.0% I, 39.8% II, and 39.2% S, whereas BR-1 isolates were 36.0% I, 20.0% II, and 44.0% S. In leaf-disc test 2 (96 isolates tested), which was based on an effective dose for 50% sporulation inhibition (ED50), most isolates were either I (44.8%) or II (51.0%), and only three (4.2%) were S. The US-1 isolates were 46.0% I, 51.0% II, and 3.0% S. Isolates of the BR-1 lineage were 45.0% I, 52.0% II, and 3.0% S. For mancozeb, the ED50 for 53 of 59 isolates was below 1.0 μg/ml. No isolate grew on medium amended with more than 100 μg of chlorothalonil/ml and the ED50 for 38 of 50 isolates was below 1.0 μg/ml. For cymoxanil, the ED50 for all 47 isolates tested was below 1 μg/ml. There is no evidence of resistance of resistance of P. infestans to protectant fungicides commonly used in Brazil.

Caracterização de isolados de Phytophthora infestans do Distrito Federal e de Goiás

“…Fato similar foi recentemente relatado para metalaxyl. Em outras regiões do Brasil, fora do Distrito Federal e Goiás, verificou-se que as duas populações do patógeno continham isolados resistentes, intermediários e sensíveis ao metalaxyl (Reis et al, 2005). Este fato também foi observado em Filipinas (Koh et al, 1994), África do Sul (McLeod et al, 2001) e Equador (Adler et al, 2004), países tropicais ou subtropicais onde a população do patógeno também é clonal.…”

“…Empregou-se o teste de independência de Chiquadrado para testar se os grupos de isolados A1 e A2 diferem quanto às freqüências de isolados resistentes, intermediários e sensíveis a mefenoxam (Reis et al, 2005). As análises foram realizadas com o programa SAS System versão 8.0.…”

Inheritance of resistance to Phytophthora infestans (Peronosporales, Pythiaceae) in a new source of resistance in tomato (Solanum sp. (formerly Lycopersicon sp.), Solanales, Solanaceae)

“…In Brazil, an estimated 20% of the production costs of tomato crops are due to the chemical control of late blight (Mizubuti, 2005). Additional production costs also occur due to the increased quantity of fungicide used or the substitution of cheaper fungicides by more expensive ones because of the emergence or predominance of resistant P. infestans strains (Reis et al, 2005).…”

Mating type, mefenoxam sensitivity, and pathotype diversity in Phytophthora infestans isolates from tomato in Brazil

“…There have been complaints about a reduction in fungicide efficacy, mainly metalaxyl, in controlling the disease in Brazil (Reis et al, 2005). The widespread use of the phenylamide class of systemic fungicides soon after their release drastically increased the frequency of resistant isolates in Europe (Dowley & O'Sullivan, 1985) resulting in failures in disease control (Bradshaw & Vaughan, 1996).…”

“…Selection of fungicide-insensitive individuals in populations of plant-pathogenic fungi is a common event and is reported more frequently for systemic fungicides. There are several reports of resistance to metalaxyl in P. infestans (Gisi & Cohen, 1996;Reis et al, 2005;Pérez et al, 2009). Thus, monitoring pathogen insensitivity, especially to systemic fungicides, is an important component of disease management (Gisi & Cohen, 1996).…”