* Citi downgrades Peru to "market weight" * Window for a recovery Colombian peso fading - JPM * New Petrobras CEO seeks to maintain previous pricing policy * Braskem jumps on M&A speculation * EM assets set for lift after choppy start to 2021 - Blackrock (Updates prices) By Susan Mathew April 19 (Reuters) - Peru's sol posted its worst session in five months on Monday after the first poll ahead of a June presidential run-off election indicated a socialist win, while most other Latin American currencies rallied as the dollar declined sharply. The sol lost 1% after Sunday poll from Ipsos Peru showed Pedro Castillo, the surprise winner of the first round vote earlier this month, in pole position to win the presidency in June, showing him with 42% support against 31% for right-wing rival Keiko Fujimori. "Fujimori's path to bridge the gap will not be easy due to her high rejection rate. We believe this opens the space for further negative repricing in Peruvian assets," said strategists at Citi Research. "We move Peru back to market weight." Colombia's peso fell after three straight days of gains. Ratings agency Fitch on Monday said Colombia's $6.4 billion tax reform, sent to Congress last week, is key for the country's bonds to retain investment grade. "Our concern is that the window for a COP recovery is fading as the debate domestically switches to fiscal reform, amid investor concerns over the country's twin deficits," analysts at JPMorgan said in a note. "If anything, risks are skewed toward the reform being watered down over the coming month, which will present another headwind for COP outperformance." As the greenback decline against most rivals, Brazil's real gained for a fifth straight session - its longest winning streak in almost a year, while Mexico's peso notched three-month highs. Data on Monday showed Mexico's economy shrank by more than 2% in March, but analysts expect a rebound in subsequent months. As copper scaled decade highs, top producer Chile's peso tacked on 0.4%. Rising copper prices have made Chile's currency one the best performing Latam units so far this year, up 1.6% compared to a 6.4% drop for Brazil's real. The BlackRock Investment Institute said on Monday the economic restart, stabilising U.S. Treasury yields and relatively cheap valuations will boost EM assets after a choppy start to 2021. With Wall Street's main indexes declining, most Latam bourses fell with Brazil's Bovespa stocks index breaking a five-day winning streak. But oil major Petrobras jumped 6% after its new chief executive said he sought to maintain policies of the previous administration, including pricing policy, which generally won market plaudits. Brazilian petrochemical producer Braskem rallied 4.7% on speculation of a potential sale of the company. Key Latin American stock indexes and currencies: Stock indexes Latest Daily % change MSCI Emerging Markets 1349.04 0.03 MSCI LatAm 2410.11 0.21 Brazil Bovespa 120863.38 -0.21 Mexico IPC 48152.41 -1.18 Chile IPSA 4985.67 -0.05 Argentina MerVal 47955.24 0.61 Colombia COLCAP 1311.36 -0.43 Currencies Latest Daily % change Brazil real 5.5554 0.53 Mexico peso 19.8387 0.38 Chile peso 699.4 0.64 Colombia peso 3625 -0.64 Peru sol 3.6718 -1.06 Argentina peso 92.9200 -0.12 (interbank) (Reporting by Susan Mathew and Ambar Warrick in Bengaluru; editing by Jonathan Oatis and Alistair Bell)