Aphids suck rather than chew. Malathion is a nerve poison. It doesn't care whose nerves it poisons. Find out what, exactly you have going on before doing anymore chemical applications.

My Italian prune leaves curl every year, and have some small, brown fungal spots. The leaves eventually dry out and fall off prematurely. I tried dormant sprays of sulphur and Kocide, but it apparently hasn't had any effect. Predictably, the tree produces almost no fruit. I don't want to get rid of it cuz the Imperial Epineuse prune right next to it benefits from the Italian Prune's pollen and I get a good crop of those. The Imperial Epineuse gets the same fungal symptoms but much less severly and never drops many leaves, although some. My Seneca Prune gets *none* of these symptoms but sady produces only small amounts of fruit, although increasing.

Here's a pic:

You should be able to see aphids during the daytime if they're there. If so, carefully spray with insecticidal soap (it typically works well on aphids).

However, if it's a fungus, and if this happens every year, you should diagnose which fungus it is and spray an appropriate fungicide, according to label directions, preventatively, as appropriate for this type of tree.

bboy's right that aphids are sucking insects and that malathion's a nerve poison. What he doesn't tell you is that, when other methods fail, a small amount of malathion works GREAT on aphids, and it is one of the insecticides labeled for many fruits.