Summer Foliar Disease Epidemics – Rust

I have received quite a few calls regarding rust and scab. Over the next few weeks, I am going to answer the commonly asked questions about these diseases and provide a management strategy to help reduce disease pressure for future years.

In regards to a general overview of Rust, please see the following:
Almond Leaf Rust – The Almond Doctor from April 10th, 2010,
Almond Rust – UCIPM Website.

I have rust all over my orchard. What can I do?
Short answer – nothing. It is too late to expect control of the disease. Once the fungus infects the leaf and produces visible symptoms, that is a clear indication that the fungus has colonized the inside of the leaf. No fungicide completely penetrates the leaf surface, which makes it impossible to cure an infected leaf.

Furthermore, spraying a fungicide upon seeing a disease epidemic will end up doing more harm than good. Since the fungus has completed it’s life cycle several times (Rust is a polycyclic disease), the population within the orchard is so high that making an application can lead to the selection of a fungicide resistant strain of rust.

If every tree has rust, what can I expect?
Defoliation. As the fungus colonizes the leaf, it begins producing spores which are the reproductive structures of the fungus. These spores are produced and move from an infected leaf to a healthy leaf. Upon complete colonization, the tree detects that the leaf is infected, forms an abscission layer which causes the leaf to fall from the tree. If enough leaves are infected on the tree, total defoliation can occur – and in most cases, is expected.

What should I do?
Since rust overwinters on the infected leaves in the form of teliospores (black spores found on the bottom of the leaves), it is important to destroy/remove all leaf litter as soon as possible. All remaining leaves should be dropped from the tree by applying 25-30 lbs/acre to zinc sulfate. By getting the leaves on the ground, the process of leaf litter breakdown can begin while the temperatures are still warm enough to encourage degradation.  Leaf litter that persists into the spring should be mowed and a small amount of nitrogen can be applied to help increase the rate of leaf breakdown.

Prophylactic sprays should begin around five weeks post petal fall, with continual applications on a monthly basis if the environmental conditions favored disease. There are a large number of products that provide control for rust, so there is no excuse not to rotate fungicide chemistries. Sulfur does an excellent job of controlling rust, and can be incorporated into summer sprays to help protect the leaves from infection. Since this product is a multi-site, it can be used in back-to-back applications. Just be careful not use it to close to any spray that contains oil. If experiencing defoliation this year, my plan for next year would include the application of a strobilurin 5 weeks post petal fall, with monthly applications of micronized sulfur through late June/early July. Next week, you will see how this plan differentiates from the management plan for scab control.

Why is rust so bad this year?
This year, the unexpected rain showers, cool weather, and higher than expected humidity provided the environmental conditions that favored rust formation. Tree density has also increased, which leads to increased humidity within the micro-climate of the orchard. Layer this on top of a large overwintering population from poor orchard sanitation from last year, and we have created the perfect environment for a rust epidemic.

My orchard floor was clean…what do you mean poor sanitation?
Leaf litter that survives the winter serves as an overwintering site for rust spores. Leaves within the crotch of the tree, caught under irrigation lines, piled up on the edges of rows, or stuck on dead branches are often found within orchards.To further complicate the issue, rust can be blown by the wind, so it is not only about your orchard being clean, but also about the cleanliness of your neighbor’s orchard next door and/or several miles away. Since we can not control all of these variables, doing our best to keep things clean and to properly time sprays should be our priority, and should provide adequate control most, if not all, years.

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