Plant growth promotion by Trichoderma species

Abstract

Trichoderma species have been studied extensively for their ability to act as biocontrol agents against a range of soil plant pathogens. These studies have led to the observation that some isolates may possess the additional benefit of direct plant growth enhancement. A commercial soil conditioning product Trichopel, which is a pellet formulation comprised of seven different Trichoderma isolates, was tested for its ability to enhance emergence, growth and production of plants under a variety of environmental conditions.

A series of glasshouse trials assessed the ability of Trichopel and its individual isolates to promote growth of strawberry and cucumber under ideal conditions. Trichopel, the individual isolates applied in pellet form, or uninoculated pellets were applied to the root zone of strawberry transplants at the rate of five grams of pellets per plant. There was no significant effect on the growth or fruit production of strawberries compared to the controls.

The emergence, growth and production of cucumbers was assessed with application of Trichopel, the individual isolates or uninoculated pellets at the rate of one gram of pellets under each seed. The rate of cucumber emergence was significantly reduced, due to vigorous colonisation of the pellet by Trichoderma or a contaminant (in the uninoculated pellet treatment), creating pathogenic activity around the young roots. In response to this result, another trial was conducted to assess the effect of application of Trichopel at different rates on emergence and growth of cucumber. At rates of one gram of pellets or more, emergence was delayed, at 10 grams emergence was completely inhibited. At one and three pellets, emergence was not significantly inhibited, however it also was not enhanced. Another emergence experiment was conducted to test the individual isolates at the rate of two pellets per seed. There was no significant difference in emergence with application of the different Trichoderma isolates compared to the control.

The effect of Trichopel or uninoculated pellets on growth of strawberry and cucumber was tested in the field, under less favourable conditions for plant growth. There was no effect of Trichopel on any of the growth or production parameters tested for strawberry plants in the field. Cucumber emergence was unsuccessful in the field. This prompted a series of investigations using incubators to control environmental conditions to test the effect of Trichopel in field soil.

The 'medium and temperature trial' tested Trichopel under three different temperature regimes (10-20 ºC, 15-25 ºC and 20-30 ºC), two different growing media (perlite and field soil) and two fungicide levels (with and without Captan dusting). Both temperature and growing medium had a significant effect on emergence. There was a strong pathogen effect in the field soil, which caused dramatic reduction in total and healthy numbers of seedlings. Application of Trichopel did not protect the seedlings from this pathogenic effect, and it did not enhance emergence or vigour by any means.

An experiment was carried out to assess if the inoculation method was responsible for the lack of effect. The mixture of the Trichopel isolates was applied as a pellet, as a powdered fungal biomass (grown in liquid molasses culture), and as a seed coating in potting mix and in field soil. Both the pellet and the powdered biomass had a negative effect on emergence and number of healthy seedlings in potting mix and in field soil. Application of the Trichopel isolates as a seed coating did not have a negative effect on emergence, rather, it significantly increased the number of healthy seedlings in field soil. In a second trial investigating the effect of seed coating with the individual Trichopel isolates there was no increase in number of healthy seedlings in field soil.

An experiment was conducted to compare the effect of the Trichopel isolates (as a mixture) with that of four Trichoderma isolates with reported growth promotion activity in the United Kingdom (T35, 20, 47 and 92, provided by Dr J. Whipps, Vegetable Research Institute, Wellsbourne, UK). This trial used the same growth system as was used in the UK trials (glasshouse lettuce and powdered biomass). Treatment with isolate 20 resulted in significantly increased leaf area and shoot fresh weight. However, Trichopel and isolates 47 and 92 significantly reduced all parameters. Root fresh and dry weights were significantly decreased in all Trichoderma treatments as compared with the control.

An investigation was carried out to identify the Trichoderma isolates comprising Trichopel using morphological and molecular techniques (RAPD and microsatellite PCR analysis). The molecular data was also used to create phylogenetic trees using two tree-making programs (PAUP 3.1 and Splitstree) to assess the genetic similarity of the Trichopel isolates. The Trichopel isolates were all identified as T. harzianum using a morphological key. Using molecular techniques the Trichopel isolates were all identified as belonging to one subgroup of T. harzianum. The Trichopel isolates fell into two groups. The first group contained T1, T2 and T5, with T5 not as closely related to the other two. The second group contained T3, T4, T6 and T7 and within this group isolates had a very high level of similarity.

The results from this thesis investigation did not verify significant plant growth promotion activity by the Trichopel isolates and only one of the four UK isolates tested. This suggests that Trichoderma species, while possibly enhancing plant growth under some circumstances, is not consistent or reliable in this effect.