My avocado trees have never done well, one has died. I have a clay soil and in some parts of my property the water after a big rain will puddle. I have some other trees that are doing well; what’s wrong with the avocados?

Avocado trees planted in a clay soil in an area of high rainfall usually, if not always, will result in unhealthy looking trees. Many trees can tolerate ‘wet feet’; the great magnolias of the South grow in the swamps. But other trees, like the avocado and citrus, do not like to sit in water. In order for trees to thrive in places like Hilo, they must either be able to tolerate lots of water or be growing in soils that have excellent drainage. Clay does not equate with good drainage.

A key element to remember is that the roots of plants need oxygen as much as they need water. When it rains, water fills the air pockets in the soil. For good plant health, a portion of that water needs to quickly drain away to allow the air to come back into the soil. Yet in clay soils this does not happen; the clay holds onto the water. In arid climates, farmers who have clay soils simply apply water less often. In tropical climates, the clay soil has little or no chance to dry out; oxygen is excluded, and the roots of the plant die. In the case of avocados, it has been shown that after 72 hours in a water-logged soil, their roots begin to die.

Once this happens, leaves commence to turn yellow, drop and eventually small branches die.

The bottom line — clay and rain are a bad combination for many plants. There is not much to be done for existing trees; you can’t change the clay and you can’t stop the rain. Next time try planting on a slope where there might be better drainage. Otherwise you can build some mounds about 3 feet high and plant on them. It won’t do any good to dig a hole, put the tree in and fill it with a different type of soil. That is like putting sand in a bathtub, plugging the drain and planting a tree in it.

I have white flies on my citrus trees. The fruit on the trees have a blackish coating on them. Can we still eat the fruit or should it be thrown away? Thank you in advance for your help.

As we’ve discussed in the past, whenever a blackish coating is detected on the leaves and fruit of the plant, it is a sign of an insect infestation. Some types of insects excrete a clear, sticky waste product which is full of sugars and amino acids.

This honeydew, as it is called, drips down to the leaf or fruit below. Fungal spores from the sooty mold fungus, which are carried by wind currents, land on the honeydew and begin to grow. The result is a black fungal growing on the surface of the plant. The fungus does not invade the plant tissue but remains on the surface feeding on the sugary substance. There is no need to spray for the fungus — deal with the insect population and eventually, the black sooty mold will wash off in the rain. The fruit can be washed and eaten as can any edible leaves.

Whiteflies can be a serious problem. In many cases, biological control will adequately manage the pest population. Controlling any ant infestations that are in the tree is essential.

For a full discussion on whitefly and their control, see www.gardenguyhawaii.com and search whiteflies.