In June 2016, olive growers in Tulare County inquired about leaf damage that appeared to be caused by insect feeding, but no pests were directly observed.  Affected leaves tended to be in the lower canopy and exhibited scalloped edges on the leaf margin (Figure 1). The location of damage within the canopy (i.e. lower leaves), the symptomatic scalloping of affected leaves, timing of symptom onset, and lack of visible insect pests suggests that damage was caused by weevils.

Two of the most commonly encountered weevils in California olive orchards are the black vine weevil, Otiorhynchus sulcatus, and the cribate weevil, Otiorhynchus cribricollis. Both are difficult to associate with feeding on olive because the adults are nocturnal and only observed with the aid of a flashlight at night. Adult weevils consume leaf margins leaving characteristic scalloped edges. Although the damage appears alarming, mature trees typically do not withstand economic damage from the insects.  Young trees, however, may benefit from management of the insect pests. 

Damage to foliage is typically observed in late spring to early summer when adults emerge, migrate up the trunk, and begin feeding on foliage. Consequently, damage is most often observed on foliage of the lower canopy and suckers. Juvenile weevils overwinter and feed on tree roots.

View the UC IPM website for a description of the pest, as well as steps that can be taken to manage high populations of weevils in olives.  The site recommends pruning lower branches and applying bands of sticky material to tree trunks to prevent the insects from crawling from the soil into the trees.   http://ipm.ucanr.edu/PMG/r583301511.html

Acknowledgement:  We thank Dr. Marshall W. Johnson for his review prior to posting.

Scalloped leaf edges in olive trees indicate feeding by adult weevils.  Photo: Elizabeth Fichtner.

Alternaria leaf spot was a focal topic of late June farm calls in humid portions of Tulare County orchards. The disease can be diagnosed based on symptoms, however, other components of the orchard ecosystem may aid in diagnosis.  The large brown leaf spots (0.5-0.75 inches) turn black (Figure 1) as the fungus sporulates on the leaf surface.  The leaf spots develop rapidly in June and July, leading to leaf drop (Figure 2) and potentially complete defoliation by early summer.

As the UC IPM Online website denotes, the disease is “most severe where dews form, humidity is high, and air is stagnant.” To assess whether environmental conditions in a block are conducive to disease, look for indications of humidity such as excessive moss or slime mold (Figure 3) on the orchard floor. Also consider whether symptoms are more prevalent on some varieties than others.  For example, in a Fritz, Monterey, Nonpareil block, the disease will be most severe on the Monterey, which is the most susceptible variety in the block.

Blocks should be monitored for Alternaria from April through June. Where the disease is present, chemical treatments may begin in late spring.  Visit UC IPM Online for more information.  Additionally, the 2015 Fungicide Efficacy and Treatment Timing table for almond can be found at the following URL:  http://ipm.ucanr.edu/PMG/r3902111.html#TREATMENT.

Figure 1. Leaf spots caused by Alternaria on almond. Photo:  Elizabeth Fichtner.

Figure 2. Infected leaves abscise and are easily observed on the orchard floor. Photo:  Elizabeth Fichtner

Figure 3. Excessive slime mold growth on the orchard floor is indicative of moist conditions. Photo:  Elizabeth Fichtner

Jim Stewart, Ag IPM Consultants, Inc., provides weekly counts of olive fruit flies (OFF) trapped in Tulare County, CA.  Trap catches were relatively high April 1-April 8, 2016, which was the first trapping interval of the season.  Traps maintained in Tonyville, Woodlake, and S. Exeter exhibited the highest trap catches of all locations monitored.   Female OFF were less prevalent in traps than males, presumably because males are actively searching for females that arrive to lay eggs in fruit.  In the San Joaquin Valley, damaging levels of infestation can occur within six days of oviposition, as the eggs quickly hatch into tunneling larvae that feed inside the fruit. 

High temperatures can have a rapid and dramatic effect on OFF populations, especially when daytime temperatures reach 95° F.  Successive days at and above this temperature causes substantial mortality of OFF adults, as well as developing larvae and eggs inside the fruit.  Also at and above this temperature, egg-laying drops off dramatically by surviving females.  For these reasons the pest tends to be more abundant in cooler California climates, such as the Sacramento Valley and along the Pacific coast. 

Inland OFF populations generally decline during the summer and resurge as temperatures cool down in the fall.  When olives reach about 10 mm in length, typically around pit hardening stage in early July, the fruit are mature enough to promote larval growth and development. Until pit hardening, OFF larvae are unable to survive in the fruit. Olive shotberries are typically not appealing for oviposition, as they are too small to support developing larvae.  Intermittent and well-timed heatwave events, such as those experienced lately in the southern San Joaquin Valley, could benefit growers battling the fly as the race to harvest approaches. 

Longevity and survival of adult female OFF increases in orchards where water and sugar sources are available.  For example, flood-irrigated orchards may be more conducive to OFF survival than orchards on microsprinklers. Similarly, orchards with heavy black scale infestation may promote OFF survival because the sugar in honeydew secretions provides a food source for the OFF.  Thus, management of water and sugar in an orchard helps to manage OFF populations.  For more information on management of OFF, visit the UC IPM Online website:  http://www.ipm.ucdavis.edu/PMG/r583301311.html

Reviewed by:  Marshall Johnson

Although over-irrigation may seem like an unlikely diagnosis after an extended drought, the foliar chlorosis observed in Tulare County walnut blocks in late May and early June are identical to  symptomology in research blocks maintained wetter than baseline (Figure 1).  In research plots, foliar chlorosis occurs within just days of an excessive irrigation event. Note that the yellowing associated with overwatering affects all of the leaves on a shoot down to the nuts (i.e. neoformed leaves that were formed in the current season). In contrast, newly emerging leaves may initially appear yellow; however, they gradually turn to green three or four leaves back from the growing tip. Over-irrigated walnut trees in research plots also have a tendency to drop one of two nut doubles, resulting in lower overall nut retention in the canopy.  When this situation is occurring, you can grasp a number of pairs of double nuts on the tree and one of each pair will often come loose with almost no force. These nuts will exhibit blackening while still on the tree (Figure 2) and squirt water vigorously when cut.   Anaerobic conditions in the root zone may cause root rot which may lead to overall tree decline and mortality.

For more information on this topic, please visit the June 2016 In a Nutshell newsletter.