Investigating decline symptoms of Super-High-Density Oil Olive in the Northern San Joaquin Valley of California

Abstract: Over 95 % of the olives grown in the United States are grown in California. In 2019, California’s total bearing acreage for olives was estimated at 37,500 acres producing 164,650 tons of olives with a total value reaching nearly $130 million. While oil olives are a growing industry in California, olive production continue to face major challenges such as unexpected freezes, droughts, pests and diseases. Over the last three years, we have received several calls from olive growers in the Northern San Joaquin valley of California reporting issues of olive trees with declining symptoms in – super-high-density (SHD) orchards – including branch wilt and dieback. In some cases, we suspected autumn freeze to be the major cause of decline. In other situations, we confirmed trees were affected by Verticillium wilt. Other dieback symptoms, associated with fungal pathogens, have occasionally been found. Overall, diagnosis has been challenging due to the strong overlap of Verticillium wilt symptoms with those of freeze injury. With SHD oil olive plantings have increased recently, most of the orchards were planted after solanaceous crops (i. e. tomato), and the occurrence of Verticillium wilt is a growing concern to the California oil olive industry. We conducted a survey of olive orchards with trees showing decline to get a closer look at the various symptoms. We diagnosed several Super SHD orchards affected by Verticillium wilt. The disease was observed in young (~ 6-year-old) trees of the three cultivars Arbequina, Arbosana and Koroneiki. On the other hand, during fall, winter, and spring, several of these new-planted orchards were at risk of injury caused by cold weather. Damage can occur at temperatures below 29 ºF depending on the age of the tree, whether the tree has had a chance to harden, the specific temperature at ground level around the tree, and the duration of the cold snap. We usually try to differentiate between injury during the growing season, which is referred to as frost injury and the freeze damage that occurs in late fall or winter. The term frost injury is restricted to damage due to freezing temperatures during the growing season while the tree is not dormant, which is due to a late spring frost. This was not the case for the past three years – based on our farm calls and minimum air temperature data (°F) collected from weather stations located near damaged olive orchards – trees were damaged with a late fall or early winter freeze, and symptoms observed were sometimes confused with Verticillium wilt disease. Symptoms were more pronounced in the Koroneiki cultivar, which may be due to its vigor. Lilac borer, commonly known as ash borer, was also recently found in young SHD olive orchards. The leaves of affected trees turn uniformly yellowish-green. The stem of those trees was girdled 1-2 feet from the ground resulting in wilting and dieback of twigs and branches, then trees eventually died.