Several calls about gummy nuts were received this week. Of concern are nuts that are oozing clear gum along the suture or side of the hull. There are many potential causes of clear gumming, which include bug damage, boron deficiency, and internal causes from an expanding kernel.
Bug damage. Most people are aware of leaf footed plant bug damage. Feeding by this insect can kill the almond embryo when the insect inserts its needle-like feeding tube into the nut. This often leaves a small pin-prick hole which results in clear gum being exuded 3-10 days after feeding. Although the risk of the bug killing the embryo decreases after shell hardening, there can be some staining of the kernel. As the season progresses, feeding by plant bugs decrease, while stink bug feeding can increase. These insects do not have long enough mouth parts to feed on the kernel, but feed on the fruit instead. They often have several feeding points on a single nut. All of these may gum. Severe feeding by stink bugs may impact kernel quality, but the economic threshold of these insects is unknown.
Boron deficiency. Too little boron can cause gumming. Usually a clear gum exudes out of the side of the nut or suture line. When cutting the nut open, the kernel is often discolored with copious amounts of gum. These nuts often drop from the tree, and if not, the gum can harden and misshape the kernels, impacting kernel quality at harvest. Another sign of deficiency is a low crop set. Tree boron status should be determined by hull analysis – which has been discussed previously. This type of deficiency is common in areas that use clean surface water and have not applied any boron to the soil.
Physiological- expanding kernels. Occasionally, almonds can gum for no apparent reason. When these nuts are dissected, there is no feeding hole and the kernel appears healthy. There is, however, a water-soaked/gummy area just outside of the shell which exudes out of a slit at the suture line or side of the hull. This type of gumming is thought to be due to a rapidly expanding kernel which is putting pressure on the almond shell and hull. As the pressure increases, the kernel breaks a small group of vascular bundles located near the shell. Although concerning, these nuts often remain on the tree and are not negatively affected.
This article focused on clear gumming which is often observed on almonds in mid to late spring. Identifying the cause of the gumming can help reduce crop loss or prevent the application of unnecessary sprays. If amber gumming is observed, which often indicates the presence of a disease, it may not be related to the causes described above. In these cases, consider reaching out to your local farm advisor or PCA to assist with identification.