By Julie Wernau

Coffee prices rose Thursday on fears that dry weather in some coffee-growing regions could lead to shortages.

Vietnam and Indonesia, the world's respective first- and third-largest growers of robusta coffee, a bitter bean variety typically found in coffee blends and instant coffee, warned that dry conditions caused by the El Nino weather event are intensifying, threatening crops there.

At least one trading firm has built up a large position in the July robusta contract, which is traded in London, and that has some investors anticipating higher prices and making the switch to the more mild-flavored arabica bean, brokers said.

Arabica coffee for September delivery rose 2.3% to end at $1.249 a pound on the ICE Futures U.S. exchange.

"If you can't grab from one pile, you go and grab from the other," said Hector Galvan, senior broker at R.J. O'Brien Futures in Chicago. "Even though they are different types of coffee, when you have shortfalls in one market, you'll see the other one rally."

Rains in Vietnam have been 50% below normal over the last 90 days, according to WeatherBELL Analytics in New York, during the country's typical wet season. Indonesia is also seeing less rains than usual.

"The robusta in Vietnam is likely to be affected and yields are likely to be down," said Joe D'Aleo, chief agricultural forecaster at WeatherBELL.

In other markets, raw sugar for October fell 1.7% to 11.27 cents a pound.

Both sugar and coffee prices have had a difficult time moving independently against the tide of the commodity plunge, Societe Generale said in a note Thursday. Sugar prices moved lower despite ideas that London-traded white sugar's recent premium against the New York raw sugar contract is a sign of increased demand.

"While there are supply-and-demand fundamental reasons for the downtrend in sugar and coffee prices, this pressure is exacerbated by the weakness in both the Brazilian real and the general commodities space," the firm said. "Continued pressure in the latter two will likely continue to keep pressure on sugar and coffee prices for the foreseeable future."

The real fell 1.5% against the dollar Thursday, encouraging producers in the world's largest sugar producing country to sell to take advantage of favorable exchange rates for their dollar denominated goods.

Cocoa for September delivery rose 0.1% to close at $3,221 a ton, cotton for September slumped 0.5% to end at 63.55 cents a pound and frozen concentrated orange juice for September ended flat at $1.228 a pound.

Write to Julie Wernau at julie.wernau@wsj.com

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