Verticillium Wilt vs. Freeze Damage of Super High Density Oil Olive

Mohamed T. Nouri, UCCE Orchard Systems Advisor, San Joaquin County, Stockton, CA

Florent P. Trouillas, Plant Pathologist, UC Kearney Research and Extension Center

Over the last three years, we have received several calls from olive growers in San Joaquin County reporting issues of olive trees with declining symptoms, including branch wilt and dieback. In some cases, we suspected autumn frost to be the major cause of decline. In other situations, we confirmed trees were affected by Verticillium wilt. Nevertheless, diagnosis has been challenging overall due to the strong overlap of Verticillium wilt symptoms with those of frost injury. Lately, we conducted a survey of olive orchards with trees showing decline to get a closer look at the various symptoms. The present newsletter reports on recent field observations and detailed symptom descriptions to help growers improve field diagnosis of Verticillium wilt vs. freeze damage in olive trees.

1- What is Verticillium wilt of olive?

Caused by the soilborne fungus Verticillium dahliae Kleb, Verticillium wilt of olive is considered one of the most important diseases of olive trees worldwide. The disease may kill trees and is difficult to control. The fungus is found throughout California Central Valley, affecting numerous species of woody and herbaceous plants in all soil types. In general, V. dahliae is categorized into two pathotypes namely the defoliating and non-defoliating pathotypes which are different in their virulence.

Symptoms and disease development. On trees affected by isolates that belong to the non-defoliating pathotype, leaves first become chlorotic (leaf yellowing), and then turn light-brown while remaining attached to branches. Symptoms usually begin in spring and slowly worsen in the early summer. Ultimately, a rapid dieback and wilting of twigs and branches takes place, especially in young trees. The bark of affected shoots may become reddish-brown. Verticillium dahliae invades and colonizes the plant's vascular system, and then infected xylem tissue becomes clogged due to fungal materials (conidial spores produced by the pathogen are translocated from the roots to branches) and host reaction substances, leading to dark vascular discolorations also referred as streaking.

The defoliating Verticillium is characterized by early drop of asymptomatic green leaves from individual twigs and branches, eventually leading to complete defoliation. In this case, trees will most likely die. Symptoms typically develop from late fall through late spring and early summer.

Verticillium dahliae is characterized by the ability to produce long-lasting resting structures called microsclerotia, free in soil or within plant debris. As thick-walled fungal structures, these microsclerotia form the primary inoculum of the disease and can endure harsh environmental conditions and long dormancy periods while waiting for favorable conditions and/or the presence of root exudates from the olive tree host to initiate germination. Germination then leads to the formation of infective hyphae, which penetrate the roots and grow within xylem vessels producing mycelium and spores. The fungus spreads to the aerial part of the plant, plugging the water-conducting system (xylem) along the way, resulting in symptom expression.

Last year we diagnosed several Super High Density (SHD) orchards in San Joaquin County affected by Verticillium wilt. The disease was observed in young (~ 6-year-old) trees of the three cultivars (Arbequina, Arbosana and Koroneiki). Most of these orchards were planted after solanaceous crops (i.e. tomato).

Field diagnosis. Knowing the field site and orchard history, keeping records of cultural practices including any chemical applications, and observing the disease pattern in the orchard are critical to provide an accurate diagnosis and implement best management strategy. When it comes to Verticillium wilt diagnosis, it is important to know the previous cropping history that may include susceptible hosts (e.g. cotton, tomatoes, potatoes, melons, etc.) and recent environmental conditions that can favor disease development. Verticillium wilt is usually favored by cool soil temperatures in the spring.

To diagnose affected trees in the field, look for symptoms of foliar chlorosis and wilting. Advanced stages of symptom development include shoot wilting and dieback, and foliar necrosis, which might be confused with desiccation associated with freeze damage. However, with Verticillium wilt, obstructed vessels become darkly pigmented and the inner vascular tissues show dark streaking following longitudinal cut into the xylem tissue using a knife. In transversal cut, infected branches may reveal spotty rings of discoloration visible mainly in the outer xylem tissue (Fig. 1). Previous olive literature on Verticillium wilt management has suggested, “Darkening/ black streaking of xylem tissue does not occur in olive wood as it does in other tree crops”. This information is no longer supported based on our recent field observations, isolation work and diagnosis efforts (Fig. 1).

olive fig 1.jpg — **Fig. 1.** Verticillium wilt symptoms in olive trees: darkening or black streaking of xylem tissue resulting in defoliation or rapid shoot death with brown leaves attached.

Ultimately, accurate diagnosis will require confirmation by a plant disease diagnostic laboratory following isolation and detection of the pathogen on the affected plant tissue. To obtain best results for diagnosis of Verticillium wilt, collect symptomatic tissues and keep the sample moist and cool until shipped to a diagnostic laboratory. Diagnosis labs also can evaluate your soil samples for the presence of microsclerotia.

Disease prevention and management. There is no effective treatment for Verticillium wilt. The most effective management strategies to protect trees are those taken before planting. Previous cultivation of V. dahliae-susceptible crops (e.g. cotton, tomatoes, potatoes, melons, etc.) in soils where olive orchards are being established has been identified as one of the major causes of disease development. The Verticillium wilt pathogen is usually present in these soils.

- A soil test to check for the presence of microsclerotia before planting is an essential way to identify disease risk. Previous research indicated that any level above 1.0 microsclerotia per gram of soil is considered risky for olives.

- Before planting: soil solarization, soil fumigation, flooding the fields during summer, growing several seasons of grass cover crops (e.g. bean, barley or sudangrass) or a combination of these treatments, are practices that may help reduce inoculum levels in the soil.

2- Freeze Damage Considerations

Super high-density oil olive plantings have increased in San Joaquin County from 3500 bearing acres in 2017 to nearly 5500 bearing acres in 2019 (San Joaquin County Agricultural Commissioner). During fall, winter, and spring, several new-planted orchards – mainly in Thornton, Lodi, and Roberts Island areas – were at risk of injury caused by cold weather. Damage can occur at temperatures below 29ºF 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.

Over the past three years, during late winter and early spring, we visited several young planted olive orchards showing symptoms of damage caused by sudden drops in temperature. 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 survey and the minimum air temperatures data (°F) collected from two CIMIS stations located near these olive orchards (Fig. 2).

olive fig 2.jpg — **Fig. 2.** Minimum air temperatures data (°F) collected from the CIMIS station located near Roberts Island area.

In these orchards, the weather was warm and mild during the preceding October and early November months, and trees continued to grow while not hardening off going into the winter. In mid-November 2018, sudden minimum temperatures were low (ranging from 26ºF to 29ºF) in some locations, causing damage of leaves and shoots particularly in young trees. Fortunately, we went through the 2019 winter with little stress to the newly established orchards, since the temperatures were warm enough to protect these trees. At the end of November and in early December 2020, similar to what happened in 2018, temperatures again dropped off at around 27 – 29ºF and were low enough to harm the trees (Fig. 2 and 3). In this case, we are dealing with a late fall or early winter freeze, and symptoms observed were sometimes confused with Vertcillium wilt disease.

olive fig 3.jpg — **Fig. 3.** Freeze damage in young olive trees produces bark splitting and discoloration in the wood. Affected branches or trees dehydrate and die back.

What does freeze damage look like?

The initial damage to look for includes tip dieback, lack of luster to the leaves, curling up of leaves as well as some necrotic or chlorotic lesions and leaf drop (Fig. 3A & B). Branches can have bark cracks and splits (Fig. 3C). For young trees in late fall, sapwood is more susceptible to cold. Sapwood is the relatively thin layer of living wood, which carries water and nutrients upwards from the roots. If killed by a freeze, damage beneath the bark will appear as brown discoloration all the way to the heartwood (Fig. 3D), and the tree will no longer be able to transport water to its branches. Damage will become noticeable in the spring or in early summer when warm temperatures lead to increased demand for water. In several cases, healthy-looking trees also showed brown wood discoloration beneath the bark tissue (Fig. 4). These symptoms of wood discoloration are different from those caused by Verticillium disease, which in most cases exhibit discolored sapwood in the recent annual rings (vascular discoloration or streaking in sapwood). With Arbequina and Koroneiki being the most commonly planted olive varieties in recent years, we noticed, based on our preliminary observation, that symptoms are slightly pronounced in the Koroneiki cultivar, which may due to its vigor.

olive fig 4.jpg — **Fig. 4.** Healthy-looking trees showing wood discoloration beneath the bark tissue due to the freeze damage.

It is still too early to tell how detrimental freeze damage is to these olive orchards (tree losses) and if the trees may later recover from this freeze injury. Assessment of tree recovery can be made later in spring to early summer when the weather gets warmer (Fig. 5).

Fig. 5. Symptoms of pronounced wilt due to the freeze damage appear later in the spring to early summer when the weather gets warmer. — **Fig. 5.** Symptoms of pronounced wilt due to the freeze damage appear later in the spring to early summer when the weather gets warmer.

Recommended cultural operations:

- A dry fall could make the freeze damage worse. Cutting back some on irrigation in September and no nitrogen applications after June could help slow down growth and may help the trees harden off before a sudden freeze event comes along. To keep orchards slightly warmer, it is advisable to run the irrigation system a few days before an expected cold snap to ensure the soil surface is moist and help the soil store a little more heat in advance during sunny days. This will also ensure trees are hydrated enough before the freeze occurs.

- Make sure trees have sufficient but not excess moisture in the soil. The important thing is to give trees an opportunity to recover.

- Prevent olive knot infection: in orchards where olive knot is present, it is important to spray copper to protect bark cracks/splits caused by freezing, ideally before subsequent rainfall occurs.

- Delay pruning until the spring when hot weather stresses the tree to see where recovery is possible as new shoots begin to develop on the trees. Branches, limbs or trunks damaged by the freeze will wilt and dieback and can be removed.

- The soil profile should be wet to the depth of rooting by the time spring growth begins. If precipitation is insufficient, irrigation may be necessary as a supplement to stimulate root activity.

- Trees severely affected by the freeze will have less total growth, which will reduce the nitrogen requirements but will stimulate vigorous regrowth similar to a heavy pruning.

- Make fertilizer decisions based on current soil reports and leaf analysis.