Brown discoloration, spotting, or russeting on lime fruit is a common concern for both commercial growers and home gardeners. Since limes are typically harvested while green, browning is rarely a sign of natural maturity. Instead, it serves as a clear symptom of an underlying stressor or disease. Identifying the specific pattern and location of the discoloration is the first step toward successful management, as the causes range widely from microscopic fungi and tiny pests to harsh environmental conditions. Addressing the root cause is the only reliable way to maintain the fruit’s unblemished green surface.
Fungal and Bacterial Causes
Many instances of lime browning are triggered by pathogenic fungi that thrive in humid, wet conditions. One of the most common is Anthracnose, caused by the fungus Colletotrichum gloeosporioides, which manifests as tear-staining on the rind. This symptom appears as reddish-brown to black streaks running downward from the stem end as fungal spores are washed down by water. Severe Anthracnose infection can also cause soft, brown decay and lead to premature dropping of young fruit.
Another significant fungal issue is Citrus Scab (Elsinoe fawcettii), which creates a distinct, rough texture on the rind. Scab lesions begin as slightly raised, corky spots that darken to a pronounced brown or black as they age. This disease attacks the fruit when it is very young. Controlling these fungal diseases often requires protective copper fungicide sprays, particularly timed to coincide with new growth and fruit development, in addition to pruning out dead wood to reduce the source of fungal spores.
Damage From Pests and Mites
Browning on the lime’s surface can also be a direct result of feeding activity from minute pests, causing a uniform change in the rind’s color and texture. Citrus Rust Mites feed on the oil cells of the fruit’s outer layer. This feeding causes bronzing or russeting, where the surface develops a rough, sandpaper-like texture and turns a dark, reddish-brown color over large areas. Although this damage is primarily cosmetic, the fruit remains edible, and heavy infestations can be managed with applications of horticultural oil or specific miticides.
Another indirect cause of browning stems from Scale Insects, such as soft brown scale. These sap-sucking pests excrete a sugary waste product called honeydew onto the fruit. Sooty mold, a black, non-parasitic fungus, grows rapidly on this honeydew, forming a thick, dark layer that makes the lime appear black or dark brown. The sooty mold blocks sunlight, reducing the tree’s ability to photosynthesize. Controlling the scale insects with horticultural oil sprays or encouraging natural predators will eliminate the honeydew, allowing the sooty mold to eventually dry up and flake off.
Environmental and Nutrient Stressors
When browning is not caused by a pest or pathogen, the issue often lies in environmental stress or nutrient deficiencies that compromise the fruit’s physiological integrity. Sunburn, or sun scald, occurs when the fruit is exposed to intense, direct sunlight, especially during periods of high temperature and low humidity. This damage appears as bleached, yellowed, or distinct brown patches on the side of the fruit facing the sun, where the cell walls have been thermally destroyed. To prevent sunburn, growers can use shade cloth or ensure the tree canopy is dense enough to provide natural protection.
Cold damage, particularly frost injury, can cause browning that is often accompanied by a watery or depressed appearance. When ice crystals form within the rind’s oil cells, they rupture, resulting in dark, water-soaked spots or pitting on the exterior surface. Limes are sensitive to cold, and protection measures like overhead watering or covering the trees are necessary when temperatures approach freezing. Furthermore, specific nutrient deficiencies can weaken the fruit’s skin; for instance, Zinc deficiency leads to ‘mottle leaf’ and can result in small fruit that is more susceptible to external damage.
Preventing Browning After Harvest
Browning can also develop after the fruit has been picked, mainly due to improper handling or storage conditions. The primary post-harvest issue is chilling injury, which occurs when limes are stored at temperatures that are too low, typically below 45 to 50 degrees Fahrenheit. Symptoms of chilling injury include surface pitting and brown or dark discoloration on the rind, which worsens with the duration of cold exposure. The ideal storage temperature for limes is around 48 to 55 degrees Fahrenheit, paired with a high relative humidity of 90 to 95 percent to prevent moisture loss.
Careful handling during harvest is also important to prevent post-harvest decay, which often appears as soft, brown spots. Any abrasions, cuts, or bruises on the rind create entry points for common post-harvest fungi like Green Mould (Penicillium digitatum). Harvesting limes gently and avoiding rough handling keeps the protective wax layer of the fruit intact, significantly reducing the chance of microbial infection. Rapid cooling of the fruit immediately after picking also slows the respiration rate, effectively delaying the onset of decay and extending the shelf life.