|My mandavilla plant has plenty of new growth on it but some of the leaves are turning yellow and ends of some new growth are dying. It gets morning and early afternoon direct sun. It is sprayed every other week with Bonide whitefly and mealybug spray|
|Yellow leaves can be sign of many things, including underwatering, overwatering, too much sunlight/heat, lack of nitrogen, or salt/fertilizer burn, to name the more common ones.
You probably noticed that fertilizers have 3 numbers on the container. These numbers refer to the percentage of nitrogen (N), phosphorous (P), and potassium (K) in the fertilizer. These 3 elements are referred to as macronutrients because plants need them in fairly large (i.e., macro) amounts to thrive. In general terms, nitrogen produces lush green growth, phosphorous helps strengthen stems and produce flowers (and eventually fruit), and potassium keeps the root system healthy. Houseplants typically need a balanced (e.g., 10-10-10) fertilizer or one with more nitrogen (e.g., 10-5-5) to promote green healthy foliage.
When leaves brown around the edges, the problem is often salt burn. Salts in the water and in fertilizer build up over time. Browning usually occurs on the old leaves first. This excess salt accumulates in the leaf edges, where it kills the tissue and the leaf dries out and turns brown. It's important to water deeply and slowly. At least once a month, water deeply enough to "leach" or push salts well below the root zone. Frequent, light "sprinklings" allow salts to accumulate in the top layers of soil, where the roots are, which is bad news. Similar symptoms occur when too much fertilizer has been applied. Always water plants thoroughly before and after applying fertilizer to help prevent burn.
Also, if plants are in too much direct sunlight, foliage can yellow and then turn brown, as it is basically "burning." Morning sun is usually fine, but the afternoon direct sun may be too intense. Finally, if the plant doesn't have an active infestation, you may want to hold off on spraying it to see if this helps. I hope this information is useful to you!