Verticillium wilt is a soil-borne fungus disease caused by the organism known as Verticillium dahliae.  Verticillium wilt can kill trees and is difficult or impossible to control and is considered a serious disease in olive trees.

SYMPTOMS

Symptoms of Verticillium wilt appear when leaves on one or more branches of the tree suddenly wilt early in the growing season; this process intensifies as the season progresses. Death of mature trees infected with Verticillium is possible. Darkening of xylem tissue, a key symptom for distinguishing Verticillium wilt in many crops, is frequently not apparent in olives.

COMMENTS ON THE DISEASE

The fungus survives from season to season in the soil and probably in the roots of infected trees. In early summer the fungus can be readily isolated from diseased tissue in infected trees. Verticillium wilt tends to be most troublesome in the southern San Joaquin Valley.

MANAGEMENT

The most effective management strategies to protect trees from Verticillium wilt are those taken before planting. When considering a new site for an olive grove, avoid land that has been planted for a number of years to crops that are highly susceptible to Verticillium wilt, such as cotton, eggplant, peppers, potato, or tomato. The Verticillium wilt pathogen is usually present in these soils.

Inoculum levels can be reduced before planting by fumigating the soil, soil solarization, flooding the fields during summer, growing several seasons of grass cover crops (especially rye or sudangrass) or a combination of these treatments. When replanting in an area where susceptible perennials were previously grown, remove as many roots of the trees or vines as possible. A resistant rootstock is not available, although some tolerance has been reported in the cultivar Ascolano.

After trees have been planted, no reliable method of control have been developed. Soil fumigation and soil solarization have provided inconsistent control in established plantings.

Soil Solarization. Beginning in late spring cover the surface of an entire block with transparent plastic that has a UV-inhibitor additive. Leave the plastic on throughout the summer and as long as practical. Inferior plastic will break down and render the treatment ineffective. Solarization gives inconsistent results when used in replant spots.

Soil Fumigation. Dry soil by withholding water during summer and using cover crops not susceptible to Verticillium wilt, such as sudangrass. The drier the soil, the better for deep penetration of methyl bromide. Deep-till the area after drying. If the soil is dusty, wait for an early rain before ripping and fumigating. Ripping a dry soil that is silty can result in large clods on the surface. Inject methyl bromide 18 to 30 inches deep with chisels and cover with gas-proof cover. Increasing the dose tends to increase the depth of penetration, but it cannot be relied upon to penetrate wet soils, especially if soils are high in clay. Do not remove the cover for at least 2 weeks and aerate 1 month before planting. Treatments may be made from late summer to early fall. In tree crops, methyl bromide often gives inconsistent control.

Girdling is a readily used practice in citrus and stone fruit orchards where it is typically used as a technique to increase fruit size.

Biennial bearing can be a serious problem in olive trees which can result in heavy crop losses to producers.  Under non-irrigated conditions, yields can fall substantially or drop to zero making the production year uneconomical to harvest.  Girdling may be a practice if done under the right conditions and timing could increase the production of an olive tree.

What does Girdling or cincturing involve?

Girdling also known as cincturing which is a horticultural practice involving eliminating up to a 1.5cm strip of bark around the trunk to stop the translocation of photosynthates to the root system.  A girdling knife or cincturing tool is used to cut the bark of the olive tree.

Researchers in Israel*  found that 15mm scaffolds covered with CaCO3 mineral oil and wax or PVC cover were most effective in facilitating wound healing.  PVC treatment was more rapid in wound healing.   During the first prevailing year after girdling found a 40% in increase yield.  Another study from Spain~ that looked into optimal widths and timing found that removing a 15mm band of bark 30 days before full bloom increased the fruit weight from 3.9g in the ungirdled trial to 4.7g.

Wounds with untreated scaffolds in the Israel study saw a drop in yield.

The width of the scaffold and healing duration as well as the time of year in which the girdling was carried out all had bearing on an increase or decrease in production.  The Israel study found that girdling olive trees increased the yield in the season following treatment with later girdling causing an increase in fruit set only.

Several scaffolds were used in the studies from 30mm to 5mm where the 5mm girdles were reported to heal too quickly and had no effect on increased production.

It was also noted that trees that were about to produce a high yield did not respond to girdling and in some cases actually reduced the yield.  Long-term studies need to be conducted to ascertain if alternate scaffold girdling should be considered.

The studies were interesting because there is potential that girdling practiced correctly in an olive grove may have a positive outcome for increased yield output however, the width of the girdle, healing timing, and time of year need to be taken into account when considering employing this technique in a commercial grove and can also have a positive effect on production in the alternate bearing years.

* STUDY

ABSTRACT

The effect of scaffold girdling on the productivity of olive trees was studied on four cultivars grown under intensive irrigated conditions. Girdling increased the yield when done in midwinter (December-February) and to a lesser extent in April. A girdling width of 10-15 mm covered with PVC tape was most effective and promoted rapid healing. Uncovered girdling wounds healed very slowly, with a resultant decline of the scaffold. Girdling increased both inflorescence formation and fruit set. With cv Manzanillo young scaffolds were more responsive than old ones. The responses of different scaffolds on the same tree to girdling were independent of each other. The degree of response of a scaffold to girdling depended on the potential yield of the scaffold in the year of treatment. No direct prevailing effect of the girdling on of the following year's yield was noted.  

Read the full study:  Girdling olive trees, a partial solution to biennial bearing. I. Methods, timing and direct tree response: Journal of Horticultural Science: Vol 58, No 2

~ STUDY

Olive tree girdling: optimum timing and width

López-Rivares, E. P. ;  Suárez-García, M. P.

Escuela Universitaria de Ingeniería Técnica Agrícola, Seville, Spain.

ABSTRACT

Six-year-old multi-trunk olive trees (cv. Manzanilla) growing under controlled irrigation in an 'off' year were girdled at monthly intervals from 15 Dec. to 15 Apr., removing 10 mm-wide rings of bark. In a separate trial, trees of the same cultivar growing under similar conditions were girdled by removing a 5-, 10- or 15-mm-wide ring of bark in Apr., i.e. a month before full bloom. Ungirdled trees served as controls. Girdling date had no significant effect on either flowering or fruit set. Removing a 15-mm-band of bark at 30 days before full bloom increased individual fruit weight from 3.9 g in the ungirdled control to 4.7 g.

Read the full study:  Olive tree girdling: optimum timing and widths.

Girdling knife (Cincturing tool) VACA
EAGTCT
$75.00(ex tax)

Rubber Tree Sealant
EATSR
$28.22(ex tax)

If you have a strange happening in your grove and it is a task for Plant Pathology, please contact directly;

[caption id="attachment_178" align="alignnone" width="300"] Dr Len Tesoriero Senior Plant Pathologist, Biosecurity Research[/caption]

Len Tesoriero is a plant pathologist who researches diseases of horticultural crops. He has a strong interest in protecting Australian horticulture's biosecurity through accurate diagnostic support and surveillance for exotic disease threats.

CONTACT DETAILS:

Dr Len Tesoriero Senior Plant Pathologist, Bio-security Research

NSW Department of Primary Industries

PH: ?02 4348 1944

FAX: ?02 4348 1910

Email: ?len.tesoriero@industry.nsw.gov.au

Website: ??www.dpi.nsw.gov.au

*** Please check with the lab for an up-to-date cost before sending your sample; it is around $150 + GST

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