In many ways our pest and disease management of fruit tree crops are exacerbated by our cultural practices.
Avocado and citrus offer some very clear demonstrations of how managing our trees can lead to reduced pesticide use. From the beginning, our selection of rootstock and scion can help lessen pest and disease problems.
In both avocado and citrus we have good rootstock selections which can handle problems, such as root rot more effectively than seedling rootstocks. Starting off with the right, healthy rootstock helps if you know that drainage will be a problem. Scion selection can have a major impact, as well. For example, ‘Lamb’ avocado is much less prone to persea mite than is ‘Hass’. This pest can significantly impact a spray program and planting ‘Lamb’ could mean virtually no sprays for this pest. There are similar examples in citrus where one variety is more prone to a pest or disease than another.
Irrigation is probably the most important cultural factor in managing tree disease. Over-, under- and improperly timed irrigations result in the conditions necessary for many root diseases. The Phytophthora spp. fungi are looking for distressed root systems brought on by waterlogging and other stressful situations. Other conditions, such as wetted trunks can also bring on some trunk diseases, like gummosis in citrus and crown rot in avocado.
Simply preventing irrigation water on the trunks can limit these diseases. Other diseases, such as black streak, stem blight and bacterial canker in avocado are bought on by soil moisture stress.
Nutrients, especially nitrogen management, have been long known to affect levels of insects, such as scale, mealy bug and aphid. Lush growth helps sustain these insects, so reducing this growth tends to lower their numbers. Managing when canopy growth occurs can affect pest severity. Avocado thrips build their populations in the spring and moves easily from leaf to fruit causing significant scarring. Promoting leaf growth at flowering time with a nitrogen application, keeps the insect on the leaves and reduces fruit scarring.
This also promotes growth to replace leaves that have been damaged by persea mite. In citrus, the incidence of citrus leaf miner damage can be reduced by avoiding summer pruning, so that a flush of growth does not occur at the same time as the population is building. Time of pruning is important in lemons to reduce the spread of hyphoderma wood rot fungus. Hyphoderma fruiting bodies are active during wet periods of rain and fog.
Pruning can change pest pressure by lowering the humidity in the canopy, introducing light and changing the microclimate supporting disease and pests. Pruning allows more thorough spray coverage, resulting in a more effective application. Modified skirt pruning can have significant effects on mealy bug and scale control, fuller rose weevil incidence, ant colonization and snail damage. It’s important that the trunk be protected as an avenue of movement for snail and ant control to get the best effects of this pruning. Copper and sticky banding can be applied to improve their control. Skirt pruning also reduces problems with such weeds as bladder pod and the ladder effect of brown rot in citrus – fungal propagules splashed from the ground onto low-hanging fruit, which in turn is splashed to higher fruit.
Keeping a canopy clean of dust and fire ash has multiple pest management benefits. Because predators are slowed in their search, they are less efficient in controlling pests. They may spend more time grooming their sensory organs, and less time searching for prey Parasites such as wasps are actually slowed by the physical abrasion to their tarsi. Dust also creates a drier environment, which is more hospitable to our pest mites.
Watering picking rows, roads and even the trees themselves can lessen mite populations. Use of cover crops can reduce dust and potentially provide pollen and nectar for predators and parasites. Of course cover crops create a whole new set of management issues, such as colder winter orchards and habitat for gophers, meadow mice and snails.
Finally harvest timing to avoid pest and disease is often overlooked. In avocado, fruit is often set in clusters.
Greenhouse thrips love the microclimate created within fruit clusters, and, if in a size-pick the cluster is reduced, greenhouse thrips will often not be a problem. Harvest timing is also important in citrus. Fruit left too long on the tree can often develop septoria fungal spot. Picking in a timely manner reduces the incidence of this disease.
These are just a few examples of how cultural practice at the right time can reduce pest and disease problems.