Control and management of lesion nematodes in apple rooted layers and nurseries

The Lesion nematode (Pratylenchus) is the most important nematode attacking apples.  They feed inside the roots causing retarded growth and low yields.  As no visible root symptoms are present, infected nursery trees may unknowingly be a great source of infecting newly planted commercial orchards.  No pesticide is registered for the control of nematodes in apple nurseries and no other management options exist to limit the spreading through infected nursery material.

Especially for applying chemical control it is very important to know what stages of Lesion nematodes occur where and at what time of the year.  As this information was not known under South African conditions observations on the seasonal occurrence over two consecutive seasons showed population peaks either in February or March.  Adults as well as larvae occurred in the roots and also in the soil during the whole growing season at rooted layers as well as nurseries.  This information was used to determine the best time to apply chemicals and also what kind of chemical to be used.

Chemical control in nurseries was usually done by leaf application with Vydate but with poor control of 30 – 40%.  A field experiment was therefore performed to compare Vydate with Nemacur both as leaf applications at the same time of the year.  Excellent control of 76% was obtained with Nemacur.  Poor control of only 52% was obtained with Vydate.

Hotwater treatments with various temperatures and different duration times were evaluated in different trials and a combination of 45°C for 30 minutes showed the best results.  Lower temperatures resulted in unsatisfactory control while higher temperatures caused a high mortality rate of rooted layers as well as nursery trees.

The immersion technique was also evaluated to control Lesion nematodes in the roots of rooted layers and good control (77%) was  obtained with 30ml of Nemacur per 100L water at immersion time of 5 hours.

During 2017  this technique was also evaluated for control in  two year old nursery trees and the dosage was increased to 50ml  per 100L water and immersion time was increased to 8 hours because of the larger root size of the trees.  Four nematicides were evaluated and Nemacur again gave fairly good control of 72% while the other three supplied control of 52% on average.

To reduce the operating time of the immersion technique the time of immersion was reduced from 8 hours to 2 hours with the same control (71%) with Nemacur.

With a third experiment Nemacur was evaluated again with trees covered totally with leaves as well as trees without any leaves.  The success of control was the same in both cases (71%).

In all the experiments the Baermann technique was used with a milk filter as usual.  In a fourth experiment the milk filter was replaced with a denser tissue paper filter and Nemacur was again used to evaluate the percentage control which increase from 71% to 99% with the change of the filters.

During 2018 the Baermann technique with two different kinds of filtering material was evaluated for three other chemicals than Nemacur.  The percentage control of Actara increased from 66,2% to 97,2%.  Nem-001 increased from only 46,4% to 86,1%.  With the results obtained during 2017 and 2018 it is clear that the immerging technique is a very promising method to manage the Lesion nematode problem for the apple industry.

Nursery trees treated with three separate systemic insecticides were kept at cold storage of 10°C for 8 weeks as is normal practise before planting in an orchard.  This treatment could possibly help to decrease the number of nematodes in the roots, however, the population increased between 59% and 94%.  At the end of the 8 weeks many new rootlets were present and could have been the reason for this unexpected increase.