Bitter rot

The primary symptom of bitter rot is the emergence of small, circular, slightly sunken spots on the fruit surface that are light brown to dark brown in color. Over time, these spots expand rapidly, eventually covering large portions of the fruit.

Concentrically arranged rings of small, salmon-colored or pinkish spore-bearing structures (acervuli) appear on the lesions. These structures are a key diagnostic feature for identifying Colletotrichum infections.

The internal tissue in the affected area becomes soft, watery, and develops a distinct bitter taste, which characterizes the disease. Under conditions of high humidity, white or greyish mycelial growth may appear on the surface of the lesions.

In cucurbits like watermelons, the disease often manifests as deep, sunken ulcers on the rind. These lesions can penetrate deep into the flesh, causing premature fruit drop or total decay of the fruit while still in the field.

During storage, symptoms may progress into a dry rot, where the fruit shrivels and becomes a hard, dark mummified structure. This renders the fruit completely unsuitable for both fresh consumption and processing purposes.

Bitter rot is a group of fungal diseases primarily caused by species of the Colletotrichum genus, most notably Colletotrichum gloeosporioides. These pathogens are classified as imperfect fungi, known for their ability to survive for long periods in plant debris, soil, and on seeds.

This group of diseases affects a wide array of crops, including pomaceous fruits like apples and pears, berry crops, and cucurbits such as watermelons. The unifying feature of this group is the development of necrotic lesions on fruits during the ripening stage or storage.

The fungal life cycle is closely linked to the phenological stages of the host plant. Spores are disseminated through rain splashes, wind, and insect vectors, entering the fruit tissue through microscopic injuries or natural openings on the fruit surface.

The infection cycle can repeat multiple times during a single season, allowing the fungus to rapidly build up biomass. A significant challenge is the latent phase, where the pathogen resides inside the fruit tissue without visible symptoms until favorable conditions trigger rot.

Effective management requires an integrated approach including good horticultural practices, the use of resistant varieties, and timely application of systemic and contact fungicides to inhibit spore germination during vulnerable stages.

Optimal conditions for the rapid development of bitter rot include high relative humidity (above 80%) and warm ambient temperatures ranging from +20°C to +28°C. These conditions are ideal for the germination of fungal spores.

Frequent rainfall, heavy mists, and prolonged dew provide the necessary surface moisture for the pathogen to penetrate the host tissue. Overhead irrigation systems are particularly conducive to the spread of the disease in orchards and vegetable fields.

Poor horticultural practices, such as high planting density and inadequate canopy management, lead to poor air circulation. This creates a favorable microclimate for fungal proliferation and spore dispersal.

Debris left in the field after harvest acts as the primary reservoir for the pathogen to overwinter. High soil moisture during the spring months stimulates the release of spores from these overwintering fungal structures.

Plants that are weakened due to nutrient deficiencies, physiological stress, or pest damage have compromised defense mechanisms, making them significantly more susceptible to infection by bitter rot pathogens.

The harm caused by bitter rot is primarily associated with direct losses in marketable yield. Infected fruits lose their quality, taste, and visual appeal, rendering them unfit for retail or long-term storage.

The disease significantly impacts the economic viability of agricultural operations by necessitating additional labor for sorting and discarding rotted fruits. Some species in this group may also produce mycotoxins, posing a risk to food safety.

In years with high rainfall, yield losses can reach 30% to 80%, requiring extensive chemical intervention and careful post-harvest management to minimize further losses during transportation and storage.

The pathogen is capable of spreading rapidly in storage facilities, moving from infected fruit to healthy ones through contact. This can lead to the total loss of entire batches, even if they appeared symptom-free at the time of harvest.

For watermelon growers, this disease restricts market access and shelf life, complicating supply chains and requiring specific transportation conditions that increase the overall production costs.

The fundamental strategy for control is the practice of strict crop rotation and the thorough removal of plant debris from the field or orchard at the end of the season to eliminate sources of the pathogen.

Timely application of fungicides during critical growth stages (blooming and fruit set) is highly effective, provided that label instructions and pre-harvest intervals are strictly observed.

• Pruning trees to improve light penetration and air circulation.
• Balanced fertilization, avoiding excessive nitrogen that promotes soft growth.
• Pest management to prevent injuries to the fruit skin.
• Selection of cultivars with known resistance to anthracnose.
• Utilization of drip irrigation to keep fruit surfaces dry.

In storage, maintaining precise temperature and humidity control is essential, alongside the sanitation of bins and equipment to prevent the spread of spores during post-harvest handling.

Regular monitoring of the crops allows for the early detection of the disease, enabling the isolation of affected areas and the removal of infected plant parts before the outbreak becomes widespread.