Evidence Points to Pesticide Run-Off 
Although the tumors on the turtle's flippers above are not said to be a direct result of only malathion, there is strong evidence that pesticide runoff is the primary reason for the cancers.

Lake and rivers are currently stressed significantly by the impact of pesticides and chemicals from 3 main sources:

Lawn Spray Chemicals (ie. Tru-Green - Chem-Lawn - etc.)
Citrus & General Agricultural Run-Off
Mosquito Control Pesticides

Fish and other aquatic organisms (including frogs & shrimp) have been reported to develop physical mutations and alterations in important survival behaviors after exposure to low level chemical exposures.

It was reported recently that 50-65% of turtles throughout sections of Florida's rivers are being found with tumors over their bodies (note the whitish growths along the front of the turtle's flippers). Although the tumors are thought to result from viral infections, the fact that only turtles in polluted waters have the tumors raises questions to the potential for pesticides and chemicals to weaken the immune systems of the turtles, thereby increasing infections by viruses and bacteria.

It is the immune system that must function properly to constantly remove virus, bacteria and cancer cells as they develop. When the immune system is weakened enough in any living mammal by chemical pesticides, viral and cancer cells can then grow more rapidly, thereby causing greater damage to the organism. 

In an article on the increased number of tumors being found on turtles, appearing in the April 21, 1996 Vero Beach, Press Journal, Florida, biologist Liew Ehrhart stated pollution is the"one common thread."
Dr. Ehrhart went on to say, 

"It only crops up in populations in degraded water, in bays, sounds and lagoons that have runoff, either urban or agricultural. In pristine water, they don't have the disease."