Managing Almond Diseases in Late Spring and Summer

Post-petal-fall and into summer can be a critical time of year to protect both the yield and quality of your harvest from diseases, particularly when rains persist and in orchards that have a dense canopy. In a presentation at the 2009 Almond Industry Conference, plant pathologist Jim Adaskaveg, UC-Riverside, reviewed the latest research on disease management techniques, and how some of the newer products fit in the management of foliar and fruit diseases. Remember, some of these products may not be registered in time for the 2010 season, so be sure to check with your crop protection professional.

The occurrence of rust is sporadic, but Adaskaveg was able to conduct trials in 2008 where the disease occurred during late April to early May. He found that the newly registered product Quash was the most effective treatment for post-infection activity against almond rust, with Abound nearly as effective in reducing both the incidence and severity of the disease.

Because scab resistance to the strobilurin Qol fungicides (FRAC resistance group 11) has emerged, fungicide resistance management should be followed in problem orchards. For these orchards, the researcher has identified a highly effective three-spray program as follows:

• A delayed-dormant spray, which delays and reduces sporulation from overwinter lesions.
  
  
• Petal-fall treatments, which also block infection from moving onto the leaves and fruit going into the growing season. For the petal-fall treatments, a number of
  multi-site fungicides were effective, including the standard Ziram. Other fungicides and approaches also worked well, including chlorothalonil (various labels). Chlorothalonil currently has a 155-day preharvest interval (PHI); a new label is being sought with a 60-day PHI, which would give it flexibility for petal-fall and post-petal-fall applications.
  
  
• For historic problem orchards, a number of single-site fungicides or premixtures have been or will soon be registered that can be used in rotation with multi-site fungicides. Fungicides applied at this later timing protect leaves and fruit post-petal-fall (two and five weeks) and into May, if rains persist. To stave off resistance, it is important that single-site fungicides be applied preventively, not once the disease is developing. Be sure to follow the UC guidelines for efficacy and suggested rotations for resistance management.

In his presentation, Adaskaveg highlighted another disease, Alternaria leaf spot, which has developed resistance to single-site mode of action fungicides when a resistance management/rotation program could not be employed. Recently this disease developed resistance to strobilurin QoI fungicides (FRAC group 11) and to carboxyanilines (FRAC group 7).

Fortunately, he reported fungicides in two different resistance groups are being registered and should be available for use against leaf spot this season. They are Quash fungicide (FRAC group 3), which has just been registered, and Inspire, another FRAC group 3 fungicide, which is expected to be registered in March. Additionally, registration of Ph-D (polyoxin-D) fungicide, a reduced-risk biopesticide, is expected in March. Ph-D is unique in its mode of action and has been placed in FRAC group 19. ”Rotations of the FRAC group 19 Ph-D and the group 3 fungicides (Inspire, Quash) will be highly effective and are welcomed,” he said.