Summer Foliar Disease Epidemics – Scab

Scab lesions found on almond twigs.

I have scab 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 (Scab 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 scab.

Scab lesions found on the underside
of almond leaves.

If every tree has scab, 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.

Symptoms of scab observed on the
top-side of almond leaves.

What should I do?
Since scab overwinters on the infected twigs in the form of lesions/fruiting bodies, population reduction is based upon controlling the disease present within the field, and coverage of susceptible tissues in the spring. Dormant applications of of copper and oil or chlorothalonil and oil have been shown to reduce summer populations of scab, and are recommended in orchards that have experienced leaf loss the previous summer/fall.

Prophylactic sprays should begin around two weeks post petal fall, with the timing window extending to five weeks post petal fall. Multiple applications for scab may be warranted during this period, but fungicide chemistries (FRAC Group #)  must be rotated. Rotating in a broad spectrum into the program may help reduce the chance of resistance, as well as lower chemical costs.  These include sulfur, Captan, Maneb, Ziram, and Chlorothalinil. Keep in mind the limitation of using some of these products either with, after, or when oil is planned to be applied.

Scab lesions found on the almond hulls.

A heavily infested orchard (defoliation on all varieties) is probably looking at 3 sprays just for scab control. This program would include a dormant application of copper/oil, 3 week post petal fall application of a strobilurin or DMI, and a five week application of a broad spectrum. The last application of a broad spectrum should also provide activity for rust. This might seem like a bit of an overkill, but scab is becoming a serious problem and growers should do the best they can to “clean up” their orchard.

Are we seeing Strobilurin Resistant Scab?
Strobiluring resistant scab populations have been documented. Many area growers have also indicated that they do not seem to achieve the same control with these products as past seasons. Until we perform an adequate sampling, this question remains.

Why is scab so bad this year?
This year, the unexpected rain showers, cool weather, and higher than expected humidity provided the environmental conditions that favored scab 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 high populations last year, and we have created the perfect environment for a scab epidemic.

Aside from this being a very abnormal year in terms of rain, we’ve got to face the fact that denser plantings are adding to foliar disease problems, and what we’re seeing is just the tip of the iceberg. Trying to address all of our disease problems with a single, in-season spray, will not always work, and will lead to resistance. In order to control these diseases over the long run, we need to apply a range of best management practices over the entire year and do more things in a preventive approach. It is concerning that we are relying on a limited range of products. As the chemical companies tell us, there aren’t too many new tools in the immediate pipeline, and the development of new chemistries take several years to complete.

In regards to a general overview of Scab, please see the following:
Tree Defoliation due to Rust or Scab – October 11th, 2010
Late Scab Timings for Almonds – April 6th, 2010
Scab: UCIPM Website.

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