Safer, Less Expensive Biological Control

BY VERONICA ANCONA, Associate Professor Citrus Center, Texas A&M University Kingsville

Emerging diseases and pathogens continue to be the main cause of yield reduction in agriculture worldwide. Unfortunately, this is also the case for citrus growers in South Texas, especially in the Lower Rio Grande Valley where incidence of Phytophthora foot rot disease is widely spread.

The disease, caused by the soil-borne pathogen Phytophthora nicotianae, reduces tree vigor, fruit yield and can even cause tree death over time.

Depending on the severity of the infection, affected trees produce less and smaller fruit, reducing tree productivity and overall yield.

To combat Phytophthora infections, fungicides are used as controlling agents. Effective chemical products reduce P. nicotianae levels in the soil, resulting in root recovery and increased fruit size. But effective chemical products can be expensive and require reapplication each growing season. Excessive use of fungicides can also increase the likelihood of pathogen resistance buildup after being preexposed to these chemicals. To avoid pathogen resistance

development, growers need to remeber to rotate products with different forms of action, apply only when necessary, minimize the number of treatments per season and always apply the doses recommended by the manufacturer.

Fortunately, integrated disease management practices can also incorporate the use of biological control agents such as Trichoderma.

Trichoderma spp. is a common fungus found within the microbial communities from cultivated soils and possesses antifungal activity. Trichoderma species have long been used as biological control agents and have shown to be particularly effective in controlling phytopathogenic fungi.

There are many commercial formulations containing Trichoderma species as the active ingredient that are now being used in vegetable production in South Texas. At the Texas A&M Kingsville Citrus Center, several Trichoderma species have been isolated from soils collected in commercial citrus orchards throughout the Texas citrus producing region.

The Trichoderma isolates collected show strong competition and inhibition to fungal pathogen growth in vitro, including Phytophthora, Alternaria and Fusarium. Besides the ability of Trichoderma to inhibit growth, some of the isolates tested were able to reduce Phytophthora spore viability to the same level as commercial fungicides.

Field studies at the Citrus Center have also shown that soil applications of Trichoderma within the drip zone of citrus trees reduce Phytophthora propagules in the soil.

The antifungal activity of Trichoderma spp. makes this group of fungi viable candidates as biocontrol agents in citrus. Organic producers can also use Trichoderma to manage fungal diseases.

At the Citrus Center, research is currently being conducted on the application of Trichoderma as part of an Integrated Disease Management program for citrus.

Based on the results from the research, the Citrus Center will develop guidelines for effective integration of disease management strategies to make recommendations to Rio Grande Valley growers.

Support for this research was provided by the Texas Citrus Producers Board.