Image © 2005 Venkat Venkataraju
Passiflora both in the wild and in cultivation are prone amongst other things to attack by nematodes, fungal, bacterial and viral infections. I have a pretty relaxed attitude to pests and other problems however and never use any pesticides or introduce natural predators. Passiflora are less relaxed however and have evolved some very sophisticated defences particularly against butterflies. A lot of us tend to expect our plants to look perfect, in the wild however many of them look an absolute mess as they are constantly being eaten. Note that some butterflies have a unique relationship with Passiflora, their caterpillars being pests and the adult butterflies feeding off the nectar and so being possible pollinators. A general observation I would make is that a happy plant is a healthy well fed plant. One of the best ways to get off to a good start is to grow your plants in clay pots, more expensive but they will pick up the electrolytes better off the surface of the clay. Note that if you live somewhere very hot the clay pots will dry out too quickly and cannot be recommended. Always feed your plants well, see Growing tips.
The variables that affect plants are temperature, sunlight, humidity, air movement, soil make-up & rainfall/watering. If all of these are similar to that which the plant would experience in its natural environment it will generally fight off any insect or disease problems. Thus in UK they are generally easy to grow in summer, but as it gets colder & darker & wetter they get further away from their ideal & will become more vulnerable to pests & diseases. The 'Black Death' gives an example of the sort of thing that can occur when a plant is grown outside its normal conditions. Even in the height of summer some insect attack is inevitable, but a plant will flower more readily when under stress. This is also why many plants flower more profusely when first bought indoors.
To your local insects the exotic combination of bright colours, perfume & nectar glands looks like an ice lolly - which they want to lick or eat & there is little you can do to stop them. I now do not use pesticides ever! Most of my plants will strike a balance with predators but if you do go down the chemical route you have to use some pretty powerful stuff, which I am against. If the plant can't hold its own, you are probably trying to grow it in conditions too far from its norm. Incidentally one of the signs of a happy plant is armies of ants marching up & down it. They are attracted by the nectar & have a protective role re disposing of other insects' eggs etc. Remember too that in the wild many passiflora are a half-chewed mess.
If you have to treat to keep pests in check, good simple treatments are to try soap solution or methylated spirits applied with a cotton bud to each bug - they don't like it! Leaf rollers (caterpillars found inside rolled up leaves) and scale can be picked off individually. More drastic treatments may be required if the plants really struggle. Treatments to help prevent or aid recovery from fungal attack (a classic sign is sudden foliage wilt from the bottom upwards) include organic Neem Oil Systemic Insect Spray & surfactants which destroy fungi without cell walls. Naiad is a proprietory surfactant available in USA, but I have had considerable success with Fairy Liquid - adding a few drops to 500ml dilute Neem solution & a pinch of fertiliser.