Last week’s rains were so welcome. However, splashing rain drops spread fungal spores and warm spring temperatures are ideal for encouraging fungal growth (powdery mildew, black spot). Spraying sulfur or copper-based fungicides to control for powdery mildew is expensive. To be effective, fungicides must be sprayed every 7 to 10 days without fail.
Powdery mildew spores can be washed off affected plants; yellowing leaves with the telltale black spots can be picked off and placed into the green waste bin, not the compost pile.
Elm and sycamore trees affected with anthracnose, another fungus spread by splashing rains, will drop their first flush of new leaves in early spring. A second flush will usually follow; in most cases the second flush of leaves will remain on the tree. UC Davis IPM does not recommend root injections of fungicides to control for anthracnose. Most trees will survive anthracnose disease and several annual losses of new leaves for years.
This drought year, monitor all your trees, especially those that have shown signs of disease, for dieback caused by drought. As the sap rises in spring, new leaves should appear on a tree from the bottom up. The top branches will be the last to flush out. Drought-caused dieback causes the topmost branches to die first. If your treetop branches don’t show new leaves soon, consider having a certified arborist check to make sure the branches are still alive and determine whether they should be removed. Dead branches can be hazardous as they fall, and removing large branches is not a job for amateurs.
Citrus should receive their second feeding of the season at fruit set which, depending on the variety, should be this month. A mature tree requires 1 to 1 1/2 pounds of actual nitrogen during the growing season, applied in thirds. That equates to about 5 pounds of ammonium sulfate. Ammonium sulfate contains only nitrogen, around 20%. Well-formulated citrus foods contain the major nutrients — nitrogen, phosphorus and potassium, plus many essential micronutrients including zinc, sulfur, magnesium, etc. The citrus food labels should provide application rates for the product.
Citrus trees are best trimmed between harvest and fruit set. Prune out any freeze-damaged wood, drought dieback, straight up suckers, and any branches that cross into the interior. Try to keep the outer leaf canopy intact and full. Bees can pollinate outer flowers more easily and the full canopy creates a sheltering microclimate and shade around the trunk.
Tickets are $30 on the day of the tour, $25 in advance. Check the Fresno County Master Gardeners’ website (http://ucanr.org/sites/mgfresno/) for ticket sales locations, maps to the gardens and printable plant lists for each garden.