How to spot and combat aspen leaf spot

A common tree in home landscapes in Idaho is the Quacking Aspen (Populus tremuloides). This attractive deciduous tree grows up to 40 feet tall. The trunk is generally long and slender but can be up to three feet in diameter. Identified by its white bark, oval leaves, and long flat petioles, the Aspen is well adapted to Idaho climates.

A common fungal disease affecting Aspens is Marssonia Leaf Spot. This fungus survives in fallen leaves and twigs infected in the past year. Spores develop in leaves on the ground and infect new leaves as they flush and expand in the spring of the following year. More common in areas with frequent rains, this fungus can infect aspen, cottonwood, and poplar trees.

Symptoms include brown flecks with yellow margins on leaves, which later turn into black spots. The Acervuli (fruiting body of the fungus) initially appears as tiny blisters in the center of spots. The fungus can invade both the leaves and twigs. Severe infections may cause defoliation in August and reduced growth. Trees will not die from the disease alone, but in this weakened state, are susceptible to other diseases, insects, and environmental stresses which may lead to the death of the tree.

Several things can be done to protect your trees. If you notice a black spot on the leaves of your trees when the leaves fall rake them into a pile and burn them. You can also avoid overhead irrigation that keeps the trees wet for extended periods of time. If you plan to plant Aspens, space them appropriately so there is space for air circulation. Prune out lower branches to improve air circulation and reduce humidity.

We live in an area where the soil is relatively high in pH. Because of that, iron is generally a limiting factor for many plants. Amending the soil with a chelated iron product can improve the health of the tree and limit leaf spot cross-over to veins.

The final choice available to control leaf spot is chemical control. Use in the spring when leaves begin to flush out and wet weather is forecasted. These applications work best before rainfall occurs. Select one of the options below as your chemical control option for the year.

• Armada 50 WDG at 3 to 9 oz/100 gal water. Do not use a silicone-based surfactant. Not for nursery or greenhouse use. Group 3 + 11 fungicide. 12-hr reentry.
• Daconil Weather Stik at 1.4 pints/100 gal at budbreak. Repeat at 7- to 10-day intervals if necessary. Group M5 fungicide. 12-hr reentry.
• Heritage at 1 to 4 oz/100 gal water plus a non-silicone-based wetter sticker. Group 11 fungicide. 4-hr reentry.
• Protect DF at 1 to 2 lb/100 gal water plus 2 to 4 oz spreader-sticker. 24-hr reentry.
• Spectro 90 WDG at 1 to 2 lb/100 gal water. Group 1 + M5 fungicide. 12-hr reentry.

For more information on chemical applications, see www.pnwhandbooks.org, select “Aspen (Populus tremuloides) – Marssonina Leaf Spot” in the search. Always read and follow pesticide labels.