Mother Nature's been far from kind to U.S. farmers lately. How's she treating the nation's biggest competition for global grain sales in South America?

Lately, in Brazil and Argentina, the weather story's been one of haves and have nots. The dry areas aren't seeing much relief, and where the rains have been falling, they continue to fall, causing some to speculate crop diseases could become a larger threat.

The last week's seen plentiful moisture across much of Argentina. That's good for the region's wheat crop, but not the best for crop diseases in general, says MDA EarthSat Weather senior ag meteorologist Don Keeney.

"Rains were abundant across eastern Buenos Aires, central and southern Santa Fe, and Entre Rios this past week, which further improved moisture supplies for wheat growth. Rains should continue to spread across much of the region this week, which should further reduce dryness in northeastern Cordoba and northern Santa Fe," Keeney says. "However, the continued abundant rains in southern Santa Fe and Buenos Aires will increase wetness concerns there as well as disease threats."

The story's way different in neighboring Brazil, Keeney says, where some spots have seen as little as half the normal rainfall in the last 6 weeks. Looking ahead, the dry trend looks to continue to pressure the region's crops.

"The drier pattern across much of Brazil will allow moisture shortages to expand further, especially across northern and far southwestern wheat areas," he says.