Summary

Phoma root rot, caused by Didymella pinodella (previously Phoma pinodella), is widespread in lentil and faba bean, but the impact on yield under field conditions in South Australia is unclear. Field trials were combined with controlled environment room studies to quantify yield loss and investigate the impact of soil moisture, temperature and pathogen load on disease expression. Yield loss in lentil was generally low, with a maximum of 12 per cent yield loss when conditions were conducive to disease.

Disease severity increased in trial plots that were inoculated with D. pinodella compared to non-inoculated plots. At some sites, disease severity increased with irrigation. However, higher disease severity did not always result in yield loss.

Fungicides reduced disease severity but did not consistently improve yield compared to untreated plots. Results show Phoma root rot can reduce yield, but losses depend on complex interactions between soil and the environment and may also be dependent on the presence of other pathogens.

Background

Phoma root rot is commonly detected in pulse crops, particularly lentil and faba bean, across South Australia. Controlled environment studies have shown D. pinodella can infect roots and reduce plant growth, but there has been limited field-based evidence linking infection to yield loss. This has made it difficult for growers and advisers to assess the economic importance of the disease or justify management interventions such as fungicides. Replicated field trials are needed to investigate how seasonal conditions, soil moisture, temperature and background pathogen levels influence crop tolerance and development of Phoma root rot.

Research Aims

The core objectives of the project were to:

• Measure yield loss in lentil and faba bean caused by Phoma root rot under South Australian field conditions.
• Develop and apply supplementary winter irrigation methods to investigate the role of soil water and spring plant available water on disease expression and yield.
• Evaluate registered and experimental fungicide products for their ability to reduce disease development and yield loss.

In The Field

Twelve field trials were run from 2021 to 2023, at Bute, Roseworthy, Loxton and Turretfield. Field trials compared lentil and faba bean plots that were inoculated with D. pinodella with non-inoculated treatments. Additional treatments examined supplementary irrigation and registered and experimental fungicides. Supplementary irrigation was applied during winter to increase soil moisture and encourage disease development.

Some sites were fumigated prior to sowing to reduce background pathogen levels and improve confidence in treatment effects. Crops were monitored throughout the season, and crop establishment, root disease severity, plant growth and grain yield were assessed.
Two controlled environment room experiments were also conducted to better understand interactions between soil moisture, temperature and the pathogen and the conditions under which Phoma root rot is most likely to impact yield.

Results

In field trials, inoculation with D. pinodella consistently increased the incidence and severity of root disease in lentil and faba bean. However, yield responses were variable between sites and seasons. In lentil, yield loss was generally low, with 12 per cent loss the highest recorded. At many sites, increased disease severity did not translate into yield reductions.

Soil type, soil temperature and soil moisture all influenced crop response to irrigation, while other background pathogens may also have explained responses to treatments in some situations. At some sites, irrigation increased disease severity, particularly in wetter and cooler conditions, but this did not always reduce yield. In some cases, higher soil moisture supported crop growth sufficiently to offset the impact of disease.

Fungicide seed treatments reduced visible disease severity in field trials. However, yield benefits were inconsistent, suggesting that reducing disease symptoms does not guarantee an economic response to fungicides. Registered fungicide P-Pickel T® only increased lentil yield in one trial at Loxton. Root DNA analysis indicated that the pathogen built up rapidly in the soil which could reduce effectiveness of single-season fungicide applications.

Controlled environment experiments indicated that disease severity was highest under cool, wet conditions, particularly when temperature was 15°C and soil moisture levels were above field capacity. When a secondary pathogen was present, higher temperatures (25°C) and water stress increased incidence of Phoma root rot, but with less impact on plant growth. These findings help explain the inconsistent yield responses observed in the field and highlight how different stressors can interact to influence the development and impact of Phoma root rot.