Potato yellowing

Potato yellowing is caused by phytoplasmas, which are specialized bacteria-like organisms belonging to the class Mollicutes. They are characterized by the absence of a cell wall and are known to be obligate parasites.

These pathogens inhabit the phloem tissue of host plants, disrupting the transport of nutrients and photosynthates from the leaves to the tubers and roots, which ultimately weakens the entire plant.

Phytoplasmas are transmitted primarily by phloem-feeding insects, most notably leafhoppers (e.g., Macrosteles laevis), which act as biological vectors.

Once an insect feeds on an infected plant, the phytoplasma undergoes an incubation period within the insect's body, after which the insect becomes a persistent carrier for the rest of its life.

Alternative hosts, including numerous weed species and perennial plants, play a crucial role in the survival of the pathogen during the off-season period.

The disease affects a wide range of economically significant plants, including potatoes, tomatoes, and various other solanaceous crops, leading to significant yield losses.

Tubers from infected plants are typically small, misshapen, and exhibit poor development. They often become necrotic or rot in the soil, rendering them unfit for human consumption.

Systemic infection leads to rapid cultivar degradation, making the seed tubers completely useless for subsequent plantings, thus posing a severe threat to potato growers.

Affected plants show a massive reduction in biomass production, as their physiological processes are redirected towards non-functional growth and abnormal development.

In severe infestation years, losses in commercial fields can exceed 50%, significantly impacting the financial viability of agricultural operations.

Symptoms become most apparent during the peak of the growing season, typically from mid-summer onwards when the insect vector populations are at their highest level.

Development and spread are strongly favored by hot and dry weather conditions, which promote the migration and rapid reproduction of leafhoppers across potato fields.

Initial infection can occur early in the spring as soon as seedlings or young plants emerge, though internal colonization may take several weeks before symptoms become visible.

The disease cycle depends heavily on the migration patterns of the vectors; therefore, late-season outbreaks often correlate with the movement of insects from maturing weeds.

Once established in a field, the disease can continue to spread until the onset of autumn temperatures, when insect activity significantly decreases.

Chlorosis of the upper leaves is a hallmark symptom, often followed by an upward curling of the leaf margins and a change in color to yellow or purple shades.

The plant exhibits stunted growth, characterized by shortened internodes and intense axillary bud proliferation, which gives the plant a distinct bushy or "witches' broom" appearance.

Reproductive organs such as flowers are often distorted, showing greening (virescence) or complete failure to produce fruit, indicating a breakdown in hormonal control.

Tubers produced by infected plants often sprout prematurely, producing thin, weak, thread-like sprouts that are unable to emerge through the soil surface properly.

• yellowing of leaf margins;
• upward curling of leaves;
• dwarfism and bushy growth;
• thread-like sprouts from tubers;
• premature plant wilting.

Effective management requires an integrated approach focusing on vector control. Systemic insecticides are recommended to reduce leafhopper populations in and around fields.

Sanitation is critical; this involves the removal of weeds around field margins that serve as overwintering sites for both the phytoplasma and the insect vectors.

The use of certified, disease-free seed tubers is the most reliable strategy to prevent the introduction of phytoplasma into new areas of production.

Crop rotation and spatial isolation from older or symptomatic potato fields can help reduce the pressure from migratory insect vectors moving between host plants.

Regular field scouting allows for the early detection and roguing of symptomatic plants, which prevents the build-up of the pathogen in the field and subsequent transmission.