Hazelnut (Corylus avellana)-Kernel Molds

Cause "Mold" is defined as any visible growth of mold either on the outside or inside of the kernel. In practice, it means any white, fuzzy mycelial growth is classified as 'mold.' Many different fungi are easily isolated from the shell or kernel, with surface sterilization, with or without any specific symptoms, before or after harvest. In Oregon, Penicillium spp., Aspergillus sp., Cladosporium sp. and Diaporthe rudis were frequently isolated from moldy kernels. Diaporthe eres was identified as the main cause of various hazelnut defects in the Caucasus region of Asia while Diaporthe foeniculina caused kernel necrosis in Chile.

Many of the following fungal problems would only be classified as decay or discoloration in the absence of active mycelial growth. Ramularia endophylla is consistently isolated from kernels with tip mold. A Phomopsis sp. and Septoria ostryae (formerly S. coryli) are commonly associated with internal discoloration. Eremothecium coryli (formerly Nematospora coryli), a yeast, is consistently associated with the disease known as kernel spot. Several other fungi have been isolated from shriveled hazelnuts. These organisms may be secondary opportunists that invade kernels that have been stressed physiologically, but all have been associated with kernels at shelling. Koch postulates have not been completed for any of the associated fungi. Insect vectors such as Lygus bugs or BMSB may be involved with yeasts.

In Oregon, moldy kernel incidence averages 0.5% to 1% annually; mold incidence in individual orchards frequently is much higher (3 to 10%). Fungi reduce kernel quality, but the degree varies with the causal agent and environmental conditions during symptom development. Mold is often highest if rains are significant in spring or during harvest. Mold can also be a problem when bins filled with nuts are grouped together and left out in the rain before drying.

The cultivars Lewis and Santiam generally have higher levels of mold than Barcelona in any particular year. Susceptibility to kernel mold is highly heritable and can be minimized when selecting new cultivars. In advanced selection trials, incidence of kernel mold was lowest for 'McDonald' (1-2%) and highest for 'Santiam' (8-17%).

Other kernel defects include: Insect injury which is generally due to filbert worm. Kernels are put in this category even if they also have obvious mycelial growth; Rancidity, which is evaluated solely by taste and not by visual characteristics; Shrivel which, is when kernels shrink and wrinkle due to moisture loss; Decay, which appear as a black (not brown) coloration that extends into the meat of the kernel; Internal discoloration of cut kernels shows a black coloration and not just brown which is considered normal.

Symptoms Most mold symptoms develop between nut maturation and when kernels are dried postharvest. The most common symptom in Oregon is a necrosis of the kernel tip where it attaches to the funiculus. Necrosis usually extends into the kernel a few millimeters. Kernel tips are blackened and shriveled, partly reducing kernel quality.

Internal discoloration of the kernel is another common symptom. Affected kernels change from a normal, opaque white to a translucent, buttery yellow and have a bitter, rancid flavor. The buttery yellow symptom is not always apparent on the kernel surface.

Lesions on the kernel surface characterize kernel spot. Lesions are dark, sunken, and varied in size and shape. All surfaces of the kernel can be affected. Kernel shriveling in combination with sporulation of fungi on the kernel surface is another defect.

Cultural control

  • Delay mechanically flailing the orchard floor until as late in spring as practical.
  • Harvest, dry, and shell kernels quickly after they fall to the ground.
  • Correctly adjust sweeper and harvester settings to reduce the amount of debris (sticks, leaves and husks) and soil (mud) that ends up in totes with harvested nuts.
  • Harvesting before fall rains has been associated with reduced mold counts. Mold counts appear to increase 2 weeks following a fall rain.
  • Totes or boxes full of harvested nuts should be kept out of the rain while awaiting processing. Any practice that increases air flow around totes will also keep additional mold from developing.
  • Some growers stack tote boxes on top of one another using an empty box on top to help keep boxes below from becoming too wet from rain.
  • Plant early-maturing cultivars.

Chemical Control There are no chemicals registered for management of kernel mold.

References Mehlenbacher, S.A., Smith, D.C., and Brenner, L.K. 1993. Variance components and heritability of nut and kernel defects in hazelnut. Plant Breeding 110:144-152.

Pscheidt, J. W. and Heckert, S. 2021. Progression of kernel mold on hazelnut. Plant Disease, 105:1320-1327. https://doi.org/10.1094/PDIS-05-20-1088-RE