Streptomycosis

These pathogens function primarily as saprotrophs in the soil, decomposing organic matter, but they can switch to a parasitic lifestyle under specific conditions.

They are gram-positive bacteria that form branched mycelium and spores, which allow them to remain dormant in the soil for several years.

Infection typically occurs through natural plant openings such as lenticels or via mechanical injuries to the root system or tuber surfaces.

The pathogen is highly persistent, making it extremely difficult to eradicate from the soil environment using conventional field sanitation methods.

The primary visual sign of Streptomycosis is the development of scabby lesions or ulcers on the surface of tubers, carrots, and other root vegetables.

These lesions often manifest as rough, corky, or wart-like growths that may vary in depth depending on the host variety and severity of the infection.

In severe cases, the skin of the affected vegetable becomes cracked and distorted, significantly detracting from its overall market appearance.

Under humid conditions, the centers of these lesions may develop a sparse, white, or grayish mycelial coating, indicative of active pathogen sporulation.

Symptoms are often localized, but if the soil inoculum density is high, the entire surface of the tuber may be covered, resulting in complete degradation.

The disease thrives in alkaline to neutral soil conditions (pH 6.5–8.0), as the pathogen is less active in acidic soils that support beneficial fungi.

Optimum temperature for the development of Streptomyces species ranges from +20°C to +30°C, coinciding with the peak growing season for most vegetables.

Low soil moisture during the initial tuber or root initiation phase is the most critical risk factor for disease development and infection intensity.

The application of fresh manure or excessive liming of the soil tends to increase the population of the pathogen and promote the incidence of the disease.

Frequent cultivation of susceptible crops in the same area without proper rotation leads to a significant build-up of the bacterial population in the soil.

The main impact is the degradation of the product's quality, making it unsuitable for commercial retail and leading to significant revenue losses.

Scab lesions serve as entry points for secondary bacterial soft rot, which causes rapid decay of roots and tubers during storage and transportation.

Yield loss also stems from stunted plant growth and reduced resource allocation to the developing tubers due to the energy spent on defensive responses.

The cost of harvesting and grading the produce increases, as significant portions of the crop must be discarded to meet quality standards.

Streptomycosis can effectively ruin the entire commercial value of a crop, turning high-quality produce into livestock feed or waste.

Effective management relies on long-term crop rotation, avoiding susceptible crops for at least three to four years in infected fields.

Soil acidification using physiologically acidic fertilizers like ammonium sulfate is a proven method to reduce the severity of the disease.

Using certified, disease-free seed stock is the most important preventive measure to ensure the pathogen is not introduced into clean fields.

• Maintain adequate soil moisture during tuberization to prevent infection through lenticels.
• Avoid the use of fresh, uncomposted manure which favors actinomycete growth.
• Select resistant varieties that show lower susceptibility to Streptomyces infections.
• Remove and destroy infected plant debris to minimize the soil inoculum level.

Chemical control involving the treatment of seed tubers or roots with systemic fungicides can provide additional protection during the early growing stages.