Highest rainfall in 18 years

The much-needed rainfall this year has already exceeded annual rainfall for the previous 18 years and we still have two months to go before we’re out of our normal wet season. As of January 17, 2023 rainfall for the current Water Year (measured from July 1 to June 30) is at 36.5 inches (927 mm) (Fig. 1). The last time this was exceeded was in 2005-06 when we had 42.2 inches (1,072 mm) of rain. A more recent “wet” year for us was in 2016-17 when we received 35.1 inches (892 mm). An important factor to consider for rainfall is the “rate,” or the amount of rain that falls over a given time frame. We got 5.25 inches (133 mm) over 24 hours on January 9 and that rate exceeded our ability to handle the water effectively.

Figure 1. Annual rainfall measured at San Luis Obispo Reservoir from 2005 to January 16, 2023. Data source: https://wr.slocountywater.org/

Heavy rains are reminders of how well agricultural fields were prepared for drainage. Low spots become painfully obvious as water will puddle and remain longer, especially in fine-textured soils (e.g., clay and silt) (Fig. 2). 

Figure 2. Flooding in poorly drained strawberry field after heavy rainfall.

Many diseases thrive during wet conditions. We’ll only mention two of the most obvious examples here: Phytophthora root rot and angular leaf spot. Phytophthora is a water mold that thrives in wet conditions. Be on the lookout for dying plants in areas where standing water has persisted for several days. Currently, the most effective treatments against Phytophthora root rot are Ridomil Gold or Orondis Gold. Orondis Gold is a new premix of mefenoxam (the a.i. in Ridomil Gold; FRAC Group 4) and oxathiapiprolin (a new a.i. in FRAC Group 49). Orondis Gold is newly registered for California strawberries.

Another disease that thrives in wet conditions is angular leaf spot (caused by Xanthomonas fragariae). This bacterial disease spreads rapidly in rain-splashed water. Look for water-soaked, angular lesions on foliage (Fig. 3). Angular leafspot will subside as rainfall does likewise and new foliage replaces old foliage. Copper sprays can be effective if applied preventatively. For this disease, the best control is sunshine. Fortunately, that’s in high supply for the moment.

Figure 3. Angular leaf spot is a bacterial disease that causes water-soaked, angular lesions that are best viewed with reverse lighting.