Phomopsis stem blight of lupin

The first symptoms appear on stems, leaf petioles, and pods as brown or grey lesions with irregular shapes, often surrounded by a darker margin.

In infected areas, the epidermis often cracks, exposing the underlying tissues. Under high humidity, the fungus produces its fruiting bodies, known as pycnidia, on the surface of the lesions.

Pycnidia appear as tiny black dots densely clustered on the infected surface. This is one of the most reliable diagnostic features of this specific disease.

Stem infection leads to premature necrosis and stem fragility. The disease often causes the entire plant to die or leads to the underdevelopment of seeds within the pods.

Early in the season, symptoms may be subtle, manifesting as a general lack of vigor, but they rapidly intensify as environmental conditions become favorable for the fungus.

The disease is caused by the fungus Diaporthe tulliensis (anamorph Phomopsis leptostromiformis). It is a specialized pathogen that primarily infects species of the genus Lupinus.

The fungus belongs to the class of Ascomycetes. Throughout its life cycle, it produces pycnidia containing alpha and beta conidia, which facilitate the spread of the disease.

The inoculum survives on infected crop residues in the soil for several years. The pathogen is highly adapted to saprophytic survival on stubble and stem debris.

A critical biological feature of this pathogen is its ability to produce mycotoxins. When ingested by livestock, these toxins cause a serious condition known as lupinosis.

The development of the pathogen occurs internally within the plant tissues, where the fungal mycelium gradually breaks down parenchyma cells, hindering normal growth and nutrient transport.

High relative humidity and frequent rainfall during the lupin growing season are the primary drivers of disease development and spread.

The optimal temperature range for rapid mycelial growth and sporulation is between 18 and 25 degrees Celsius.

Dense crop stands, which restrict airflow and maintain a humid microclimate in the lower canopy, are highly susceptible to rapid infection spread.

Conidia are dispersed primarily by rain splash and wind, allowing the pathogen to move quickly across large areas of the field during periods of wet weather.

Short crop rotations that return lupins to the same field too frequently lead to the buildup of a significant inoculum reservoir in the soil.

The primary economic damage caused by Phomopsis stem blight is the significant reduction in seed yield and seed quality, with affected seeds showing low germination rates.

Plants infected with the pathogen can contain dangerous levels of mycotoxins, making both the grain and the green biomass unsuitable for animal feed.

Severe stem infection often leads to lodging, which significantly complicates mechanical harvesting and results in substantial seed loss.

The disease impairs photosynthetic activity, leading to premature senescence of the plants and a decrease in protein content within the crop.

In epidemic years, yield losses can range from 30 to 70 percent, rendering the cultivation of susceptible lupin varieties economically non-viable.

The primary strategy for management is the use of high-quality, disease-free seed and the application of systemic fungicide seed treatments before sowing.

Maintaining proper spatial isolation between lupin crops of different years and implementing sound crop rotation practices are essential for reducing infection pressure.

• Deep tillage to bury infected plant residues.
• Cultivation of resistant or tolerant lupin varieties.
• Timely foliar fungicide application during critical growth stages.
• Removal of alternative host weeds that may harbor the fungus.

Chemical control programs should be based on systematic field monitoring and local weather forecasting to ensure fungicides are applied at the most vulnerable stages.

Ensuring optimal plant density is crucial to improve canopy aeration, which reduces humidity levels around the lower stems and creates an unfavorable environment for fungal development.