Thickened seedling

The fungus invades meristematic tissues, leading to the disruption of plant growth hormones. This results in the characteristic thickening and hypertrophy of stems and leaves.

Under a microscope, the presence of fungal hyphae in the intercellular spaces confirms the infection. The pathogen relies on haustoria to extract nutrients from the host plant.

It is important to distinguish this condition from mechanical injury or other physiological disorders. The presence of a white, powdery fungal bloom is a definitive diagnostic sign.

Without intervention, the pathogen will continue to develop within the plant tissues, eventually leading to the formation of spores that spread to nearby healthy seedlings.

This condition primarily affects members of the Ericaceae family, such as blueberries, cranberries, and ornamental azaleas or rhododendrons during their early growth stages.

The damage caused by the pathogen includes the disruption of the vascular system. This prevents the normal movement of water and essential nutrients throughout the seedling.

Infected seedlings exhibit stunted growth, structural weakness, and a loss of aesthetic value in ornamental nursery production. Economic losses can be substantial in greenhouses.

The deformed tissue becomes soft and succulent, making it susceptible to secondary bacterial and fungal infections. This often leads to total plant death.

Seedlings affected early in development rarely recover sufficiently to become healthy, productive plants, often requiring immediate culling.

The disease typically manifests during periods of high humidity and moderate temperatures, which are essential for fungal spore germination and development.

Spring and early summer are the most critical times for the spread of Exobasidium. Rain and irrigation water droplets facilitate the transmission of spores between plants.

Temperature plays a vital role in disease progression, with optimal growth occurring between 15°C and 22°C. Cooler conditions can slow down the development but rarely eradicate the pathogen.

In greenhouses, poor ventilation and high humidity significantly increase the likelihood of an outbreak. Continuous moisture on the foliage is a key trigger for sporulation.

Outbreaks can occur rapidly, often within a few weeks of favorable environmental conditions, making regular scouting of seedling trays essential.

The primary symptom is the visible thickening or swelling of the seedling stem, often appearing spongy or fleshy in texture compared to healthy tissues.

Leaves may show severe distortion, curling, and a change in color to a pale yellowish or reddish hue. This is due to the alteration of chlorophyll production.

A thin, whitish powdery layer forms on the surface of the deformed parts. This layer contains the fungal spores that are released into the air.

Stems may become twisted or bent, causing the seedling to collapse under its own weight. This is a clear indicator of severe tissue damage.

• Hypertrophy of stems and petioles.
• Distorted and discolored leaves.
• White powdery fungal growth.
• Growth cessation and stunted development.

The most effective management strategy is strict sanitation. All infected seedlings must be removed and destroyed immediately to break the pathogen's life cycle.

Maintaining optimal greenhouse conditions is critical. Ensure good air circulation, lower the humidity levels, and avoid overhead irrigation where possible.

Fungicides can be applied as a preventive or curative measure. Products containing copper or specific systemic fungicides are often effective if applied early.

Prior to planting, sterilizing the growing medium and disinfecting trays helps to minimize the risk of primary contamination from the soil or infrastructure.

Regular inspection and early detection are key to preventing the spread. By removing symptomatic plants before sporulation occurs, nurseries can significantly reduce potential losses.