January's checklist of garden chores (besides pruning) isn't a long one. Most of the work is preventative, and will lessen or prevent diseases and insect problems during the next year's growing season.
Many insects (whiteflies, aphids, scales) lay their eggs to overwinter on garden debris. Others lay eggs (grasshoppers) or overwinter as grubs (cutworms, hoplia beetles) in undisturbed, uncultivated soil or weed patches. Mummies or dried fruits and vegetables left hanging on the tree or vine harbor fly eggs. Fungal spores (anthracnose, black spot, powdery mildew) remain inactive on fallen leaves and twigs in winter, then proliferate during warm, wet spring weather, spread by splashing rain and overhead sprinklers.
A thorough clean-up of fallen debris and mummies along with pulling weeds and cultivating bare soil with a wiggle hoe can really reduce pest insect populations.
Horticultural oils are the second line of attack agains pest insects; the oil smothers insect eggs that were laid to overwinter in bark cracks, trunk crotches and leaf nodes. Spray winter-weight horticultural oil in January to drench bare branches and wood, especially on those trees or bushes that have had insect problems in the past year. (Summer-weight oil, sprayed in May to smother the immature, crawler stage of scale insects doesn't adhere well in wet weather).
UC Davis Integrated Pest Management does not recommend fungicide treatments for anthracnose on sycamores, elms or most ash trees. Fungicide sprays can be used to treat for anthracnose on Modesto ash. Treatment for anthracnose is limited to preventing the spread of the spores by splashing water. Replace overhead sprinklers if necessary, and make sure to clean up all tree litter regularly.
The systemic insecticide, imidacloprid, (Bayer's "Merit" is one brand) is very effective in eliminating severe aphid and scale infestations in landscape trees (crape myrtles, hackberry). It is applied as a soil drench in January when the trees are dormant to be carried up into the leaf canopy as the sap rises in spring. Imidacloprid is a neonicinitoid or neonic insecticide; neonics are suspected as one cause of colony collapse disorder which is killing off the bee population worldwide. The systemic enters into all plant tissue, including the pollen, which is eaten or carried back to the hive by the bees. Before applying imidacloprid, consider whether other less toxic or non-toxic means of control could reduce pest insect populations to a tolerable level. Horticultural oil sprays, applied in winter when they are less likely to affect the bees, or washing aphids off small trees often, or encouraging the presence of beneficial insects that prey on pest insects should be tried first.