Studies on the biocontrol of seedling diseases caused by Rhizoctonia solani and Pythium sp. on sorghum and tef.

Abstract

Rhizoctonia solani and Pythium spp. are aggressive soil-borne fungal pathogens responsible

for seed rot and seedling damping-off of many crops. With increased environmental and

public concern over the use of chemicals, biological control of these diseases has been

attracting more attention. However, success with this strategy depends on the development of

effective antagonists, which requires repeated in vitro and in vivo tests.

Bacillus spp. were isolated from a soil sample obtained from a field where sorghum and tef

had been grown for at least two years. Potential Bacillus isolates were screened for their

ability to inhibit in vitro growth of R. solani and Pythium sp. Among 80 isolates tested,

endospore forming Bacillus spp. H44 and H51 gave highest antifungal activity against the two

test-pathogens in three consecutive tests. Results demonstrated that both H44 and H51 have

potential as biocontrol agents against diseases caused by these two pathogenic fungi.

The interaction between three isolates of Trichoderma (T. harzianum Eco-T, Trichoderma spp.

SY3 and SY4) and Pythium sp. were investigated using in vitro bioassays together with

environmental scanning electron microscopy (ESEM). Visual observation on the dual culture

tests revealed that hyphal growth of Pythium was inhibited by these antagonists soon after

contact between the two organisms within 3-4 days of incubation. The ESEM investigations

showed that all three isolates of Trichoderma grew toward the pathogen, attached firmly,

coiled around and penetrated the hyphae of the pathogen, leading to the collapse and

disintegration of the host's cell wall. Degradation of the host cell wall was postulated as being

due to the production of lytic enzymes. Based on these observations, antibiosis (only by Eco-T)

and mycoparasitism (by all three isolates) were the mechanisms of action by which in vitro

growth of Pythium sp. was suppressed by these Trichoderma isolates.

The reduction of seedling diseases caused by R. solani and a pythium sp. were evaluated by

applying the antagonists as seed coating and drenching antagonistic Bacillus spp. (B81, H44

and H51) and Trichoderma (T. harzianum Eco-T and Trichoderma spp. SY3 and SY4). On

both crops, R. solani and Pythium sp. affected stand and growth of seedlings severely. With

the exceptions of H51, applications all of isoltes to seeds reduced damping-off caused by R.

solani in both crops. Application of Eco-T, H44 and SY3 to sorghum controlled R. solani and

Pythium sp. effectively by yielding similar results to that of Previcur®. On tef, biological

treatments with Eco-T and SY4 reduced seedling damping-off caused by R. solani and

Pythium sp., respectively, by providing seedling results similar to the standard fungicides,

Benlate® and Previcur®. Most other treatments gave substantial control of the two pathogens

on tef. Overall, Bacillus sp. H44 and T harzianum Eco-T were the best biocontrol agents from

their respective groups in reducing damping-off by the two pathogens. In all instances, effects

of application method on performance of biocontrol agents and adhesive on emergence and

growth of seedlings were not significant.

A field trial was conducted at Ukulinga Research Farm at the University of Natal,

Pietermaritzburg, South Africa, to determine efficacy of biological and chemical treatments on

growth promotion and reduction of damping-off incited by R. solani and Pythium sp., and to

evaluate the effects of a seed coating material, carboxymethyl cellulose (CMC), on seedling

emergence and disease incidence. Seeds of sorghum and tef were treated with suspensions of

antagonistic Bacillus H44 or T harzianum Eco-T, or sprayed with fungicides, Benlate® or

Previcur®. Application of Benlate® and Previcur® during planting significantly increased the

final stand and growth of sorghum seedlings. Seed treatments with both H44 and Eco-T

substantially controlled damping-off caused by Pythium, resulting in greater dry weights of

seedlings than the standard fungicide. However, they had negative effects when they were

tested for their growth stimulation and control of R. solani. The CMC had no significant effect

on germination and disease levels. These results showed that these antagonists can be used as

biocontrol agents against Pythium sp. However, repeated trials and better understanding of the

interactions among the antagonists, the pathogens, the crop and their environment are needed

to enhance control efficiency and growth promotion of these antagonists.

Some of these biocontrol agents used in this study have the potential to diseases caused by R.

solani and Pythium sp. However, a thorough understanding of the host, pathogen, the

antagonist and the environment and the interactions among each other is needed for successful

disease control using these antagonists.