Gordon Johnson, Extension Vegetable & Fruit Specialist; email@example.com
Each year we see problems with cantaloupe leaves turning yellow. There are several potential causes. If the yellowing is on leaf edges it most commonly is due to salt effects and fungicides, see the article by Jerry Brust two weeks ago for more details http://agdev.anr.udel.edu/weeklycropupdate/?p=4562. Copper fungicides are often the culprit in this leaf yellowing, causing a phytotoxic reaction. Foliar fertilizer applications can often worsen the yellowing by increasing salt levels on the leaves.
Each year there are some fields of cantaloupes that are affected by manganese toxicities. This occurs when bed pH drops below 5.4 which affects soil chemistry so that plant available manganese increases greatly and plants take up quantities that become toxic. As a micronutrient, Manganese is needed in only small amounts and the sufficiency range is between 20-100 ppm. Magnesium deficiencies also can occur at low pH and older leaves will show interveinal chlorosis. These symptoms can be confused with mite damage so check for mites in the diagnostic process.
Air pollution is another cause of yellowing of cantaloupe leaves. This yellowing is usually confined to older crown leaves.