Black spots on olives are common for growers and home gardeners. These discolorations raise questions about tree health and fruit quality. Understanding their causes is important for proper olive cultivation and assessing fruit condition. This article clarifies common reasons for black spots on olives.
Common Causes of Black Spots on Olives
Fungal Diseases
Fungal pathogens commonly cause black spots on olives, notably Peacock Spot and Anthracnose.
Peacock Spot, caused by Spilocaea oleagina, causes small, circular, dark spots on olive leaves, sometimes appearing on green fruit. These spots often have a pale halo, resembling an eye. The fungus thrives in cool, wet conditions, leading to defoliation and reduced fruit production.
Anthracnose, primarily caused by Colletotrichum species, causes sunken, dark, soft lesions on ripening olives. Spots enlarge, covering fruit and leading to rot. Damaging in warm, humid environments, it reduces olive oil quality by increasing acidity and affecting flavor. Spores disperse by rain and wind, infecting fruit through wounds.
Insect Pests
The olive fruit fly, Bactrocera oleae, is a key insect pest causing black spots. Adult female flies lay eggs just beneath the fruit’s skin. Larvae hatch and tunnel through the pulp, creating internal damage often unseen until fruit is cut.
Feeding breaks down fruit tissue, causing external discoloration or sunken black spots. Damaged areas are susceptible to secondary infections, contributing to blackening, decay, and premature fruit drop.
Bacterial Issues
Olive Knot, caused by Pseudomonas savastanoi pv. savastanoi, causes dark discolorations and deformities on olive trees and fruit. It enters through wounds (pruning, hail, insect damage). Inside, it stimulates abnormal cell growth, forming galls or knots.
Though primarily on branches, galls also develop on fruit. They appear as raised, dark, rough, black or dark brown swellings, malforming fruit or making it unmarketable. The disease is more prevalent in high humidity and during rain.
Environmental Factors
Environmental factors also contribute to black spots on olives. Bruising, mechanical damage, occurs when olives are impacted during harvest, transport, or by strong winds. Trauma damages fruit cells, leading to oxidation and black or brown discoloration.
Sunscald occurs when olives are exposed to intense, direct sunlight, especially in hot periods. Exposure burns skin and tissue, resulting in sunken, leathery, dark spots. These spots are typically found on the sun-exposed side and can vary in size. Sunscald can also make the fruit more susceptible to secondary infections.
Assessing Olive Edibility
The edibility of black-spotted olives depends on the cause and extent of damage. Minor cosmetic damage (slight bruising, small Peacock Spot lesions) often doesn’t affect olive quality or safety. These olives can still be processed, though their aesthetic appeal might be reduced. If spots are superficial and flesh firm/healthy, fruit is likely safe after curing.
Olives with severe black spots, especially from extensive insect infestation or significant rot, are generally unsuitable for consumption. Heavily infested olives may show internal decay and secondary infections, making them unpalatable and unsafe. Similarly, olives with large, soft, or moldy lesions from Anthracnose indicate widespread decay and should be discarded. When in doubt, avoid consuming olives with extensive or questionable damage.
Prevention and Management Strategies
Effective management of black spots starts with good cultural practices promoting tree health and minimizing disease/pest susceptibility. Adequate tree spacing improves air circulation, reducing humidity and inhibiting fungal growth. Proper sanitation (removing fallen leaves and fruit) helps break pathogen/pest life cycles. Balanced fertilization and irrigation contribute to tree vigor and resilience.
For fungal diseases (Peacock Spot, Anthracnose), proactive measures are key. Pruning to maintain an open canopy improves air circulation and light penetration, reducing fungal development. Copper-based fungicides can be applied preventatively, especially during high humidity or rainfall. Selecting disease-resistant olive varieties is also a long-term strategy.
Managing insect pests, like the olive fruit fly, involves monitoring fruit for infestation with traps. Early detection allows timely intervention (mass trapping, kaolin clay application). For bacterial issues like Olive Knot, prune infected branches carefully; sterilize tools between cuts to prevent spread. Avoiding tree wounds during cultural practices also prevents bacterial entry.
Addressing environmental factors involves careful olive handling during harvest to prevent bruising. Harvesting techniques minimizing physical impact reduce post-harvest black spots. For sunscald, ensure sufficient canopy density provides natural shade during intense summer heat, possibly through strategic pruning.