As the interest in growing fruit and ornamental trees organically rises, the challenge of controlling pests organically is becoming apparent. Areas that have been treated heavily with non-organic sprays take time to rediscover their biological balance, and the influx of pests can seem overwhelming. But there are organic tree sprays and techniques that can smooth the transition from chemically grown trees to organically grown trees.
Trees are sprayed to prevent insect damage and to prevent fungus. Insecticides are often oil-based to help smother insects, but risk "cooking" tree foliage if the weather is too hot. Organic fungicides are copper- or sulfur-based. Biological microbe sprays are solutions with tiny organisms which infect pest species, but leave all other species, including humans, unaffected. The USDA's Agricultural Research Service (ARS) has also had success with spraying compost tea, a strained dilution of compost, on trees to increase their natural insect resistance and control disease.
Oil-based sprays should be applied in the spring before temperatures reach 70 to 75 degrees Fahrenheit to avoid leaf burn. Sprays containing microbial biological controls such as the bacteria Bacillus thuringiensis must be applied at the right temperatures in order for the organism to survive. Fungicides should be sprayed on trees as a preventive measure, as most fungi cannot be treated effectively once they've established themselves in the tree system. Compost tea was applied to nut trees by the ARS once per month after flowering.
Many labels for organic pesticides have a stop date for applying before harvest. This is to prevent substances that are potentially harmful to humans from being consumed. There are very few organic controls with this stipulation, and pyrethrins are even added by commercial growers just before harvest so that fruits aren't vulnerable to insects during shipping. But no matter how safe the substance you spray on your trees, always wash the fruits, nuts or leaves thoroughly before consuming to rid the food of spray residue.
Adding dish soap to some fungicides and insecticides, including Bacillus thuringiensis sprays, breaks the surface tension on tree leaves, allowing the solution to cover the surface and stay on longer. If you bank tropical trees over the winter (adding soil around the base and up to the lower branches to prevent freeze damage), spray the trunk down thoroughly with organic fungicides before banking.
Deciding to switch from a chemical tree care regimen to an organic one will involve a period of transition. It takes time for insect predator populations to build up after long-term, broad-spectrum insecticides have been used. Releasing predatory insects, planting a variety of tree species and companion plants, keeping the soil fertile with compost, removing and destroying infested fruits, and watching your trees carefully for signs of trouble before it gets out of hand will help smooth the transition period.