Tree Defoliation due to Rust or Scab

Due to the wet year, foliar diseases have been problematic. In many cases, growers have observed late season defoliation due to rust (Tranzschelia discolor f. sp. dulcis) and scab (Cladosporium carpophilum). These two foliar fungi initially infect the tree in mid to late spring, reproducing within the canopy of the orchard, killing leaves by late summer/early fall. The fungi must be controlled before the infection occurs to prevent late season defoliation.

To help prevent an increase of rust and scab for the coming growing season, the following practices are recommended:

Rust:

Rust symptoms on almond leaves.
Note the brown spores on the underside of the leaf.

Orchards that have defoliation due to rust should have zinc sulfate (20-40 lbs/acre) applied in late October/early November. This practice will hasten leaf fall. Once the leaves are in contact with the soil, the leaves will begin to break down, reducing the overwintering inoculum. The rust fungus is not a soil inhabiting fungi and will not survive without leaf tissue. It is also important to remove any leaves from the crotch of the tree. These leaves serve as an overwintering reservoir and infect leaves the following spring.

Fungicide sprays should be considered to help control another outbreak the following year. Orchards with a history of rust should apply sulfur or maneb 5 weeks after petal fall and follow 4 to 5 weeks later in late spring and summer with a FRAC Group number 11 fungicide (strobilurin) to control leaf infections. To be effective, fungicides must be applied before rust symptoms are visible.

Scab:

Scab lesions on almond twigs. These
lesions are fruiting bodies that can re-infect
 the tree the following spring.

Orchards experiencing defoliation due to scab should have a dormant application of either copper/oil or liquid-lime sulfur. These applications will help reduce and/or delay the sporulation of twig infections in the spring. Sprays can be made as late as the delayed dormant stage, in which copper/oil applications have been shown to be the most effective. Orchards with susceptible varieties (Carmel and Fritz) that experienced defoliation should be treated in the dormant period.

Fungicide applications in the spring should be made between 2-5 weeks after petal fall. Scab resistance to quinone outside inhibitor fungicides (i.e. strobilurins) has been documented. Rotation of fungicides when spraying for scab should be considered.

For further information, please see the UC IPM Pest Management Guidelines.

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