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California: Rain Brings Good Spirits and Root Rot Fears

February 24, 2017

CALIFORNIA — So far this year, Mother Nature has been relentless. Storms across California have dropped record amounts of precipitation on many tree nut, citrus and stone fruit acres. And because California soils are naturally infested with various species of Phytophthora, which thrive in these conditions, we’re looking at a high-risk Phytophthora season for tree nuts, citrus and stone fruit.

Here are 5 situations where Phytophthora infection is most likely to occur, and the factors that make it such a devastating pathogen in permanent crops:

  1. Wet weather in the spring. Major root flushes and leaf growth occur before and during spring bloom. Wet weather during this time can increase the activity of Phytophthora.
  2. Saturated soils and/or standing water. This creates the perfect scenario for infection to occur and also limits needed oxygen within the root zone.
  3. Good land practices. Years of drought may lend some to think root rot is not possible, but pulse irrigation, salty well water and micro irrigation magnify the issue even in summer or dry winters – often this can make it worse than times of ample surface water.
  4. Permanent crops. In permanent crops, we have no ability to replant or correct mistakes that lead to root rot, so disease impact is much greater than in crops like tomatoes or vegetables. This also means permanent crop growers can deal with the devastating effects of root rot for years to come, especially if it is overlooked for some time.
  5. Micro irrigation vs. flood irrigation. Let’s talk about this a little further. The movement to micro irrigation has made us very efficient in water use, but it has created a perfect scenario for root rot to flourish. Let’s compare:
    • With flood irrigation, we irrigate with large amounts of water and then come back 2 or 3 weeks later with more water. This allows the soil to dry down, and free standing water is not present on a continual basis.
    • With micro irrigation, we apply small amounts of water weekly with an irrigation often happening over 3 or 4 days. This means that every week we have water running for 3 or 4 days at a time, thus keeping the top 1 foot of soil (where 90% of the roots are) in a state of saturation. This creates perfect conditions for root rot to occur. We’ve become more efficient, but have brought Phytophthora back to the front line of damaging root health.

What can you do to prevent Phytophthora? Because strong, healthy root systems are essential to overall tree health and productivity, it’s important that growers establish an aggressive integrated management system for Phytophthora that includes:

  • Cultural practices such as providing favorable soil conditions, proper irrigation, using certified disease-free nursey stock and preventing soil/water movement from infested areas
  • Soil sampling and testing
  • Soil-applied fungicide during root flush (near bud break/leaf emergence) in the spring and fall

Ridomil Gold® SL fungicide offers long-lasting disease protection against crown rot, collar rot and root rot caused by several Phytophthora species. According to research results from a combined effort of both Dr. Jim Adaskaveg at the University of California Riverside and Syngenta, trees showed larger root mass, tree weight and tree diameter when using Ridomil Gold SL in a Phytophthora management program.

Additionally, the Soil Pathogen Assessment (SPA) program from Syngenta helps make soil sampling easy and efficient. Contact your Syngenta representative to learn more about soil sampling through the SPA program: www.Syngenta-US.com/rep-finder.

Submitted by Garrett Gilcrease, Agronomy Service Representative, Hanford, CA, Syngenta