Mold in walnuts

Growing walnuts is a rewarding venture for farmers, but it is also a challenge. One of the most serious problems facing walnut growers is the emergence of mold, which can significantly affect the quality and yield of the crop. In this article, we will review the causes of mold on walnuts, contributing factors, effective management strategies and prevention of this problem based on recent research.

Research has shown that several fungi are responsible for walnut mold and husk rot. These mainly include Fusarium, Alternaria, Botryosphaeria, and Phomopsis. They are distinct from walnut blight, which is caused by the bacterium Xanthomonas arboricola. These fungi can enter the walnut through various vulnerabilities, such as sunburn, insect damage, and natural openings in the stem of certain walnut varieties. In addition, certain environmental and management factors can also exacerbate the problem. 

• Excessive sun exposure weakens the nut's protective outer layers, making it more vulnerable to fungal infections
• Pests create entry points for fungal spores to penetrate
• The fungi often attack during or immediately after cracking of the husk, when the nut's protective barriers are compromised.

To effectively control mold in walnuts, researchers and growers recommend a combination of proactive management practices and targeted treatments.

                     ·       Fungicide application

Fungicide applications made before and during early hull split have been shown to reduce fungal infections that lead to mold. Fungicides such as Merivon and Rhyme, applied three weeks before cracking of the green fruit, are particularly effective against moulds such as Fusarium and Alternaria. However, late fungicide applications are less effective, highlighting the importance of timing.

·       Timely harvesting

Harvesting walnuts quickly and harvesting them promptly after the husk has cracked are essential to minimize the time walnuts are exposed to potential infection.

·       Integrated pest management

Controlling insect populations reduces the risk of physical damage to walnuts, thereby limiting entry points for fungi.

·       Protection from the sun

Measures to protect walnuts from excessive sun exposure, such as using protective covers, can help reduce vulnerability associated with scald.

Fungal mold and husk rot in walnuts is a complex and persistent problem that requires a proactive and informed management approach. By understanding the causes and contributing factors—such as environmental stress, insect damage, and hull vulnerability—growers can implement effective practices to reduce the risk of infection. A combination of timely fungicide application, integrated pest control, and proper orchard management provides strong protection against mold and husk rot. With the right strategies in place, producers can maintain both the quality and profitability of their walnut crops.

One of the most important actions to be taken to control mold is the appropriate drying and storing of the nuts.