Husk spot

Pseudocercospora macadamiae

Causal Agent:

  • Fungal Pathogen: Pseudocercospora macadamiae is a fungal pathogen that causes leaf and nut spot disease in macadamia trees (Macadamia integrifolia and Macadamia tetraphylla).

2. Symptoms:

  • Leaf Spots: The most common symptoms include the development of small, dark brown to black spots on the leaves.
  • Lesion Enlargement: Lesions may enlarge, coalesce, and result in extensive blighting of the leaves.
  • Nut Spotting: In severe cases, the fungus can also cause spotting on the nuts, affecting the quality of the macadamia harvest.

3. Disease Cycle:

  • Overwintering: The fungus may survive in infected plant debris, serving as a source of inoculum for new infections.
  • Infection: Spores (conidia) produced on infected tissues are spread by wind, rain, or physical contact, initiating new infections.
  • Favorable Conditions: Warm and humid conditions, along with rain or high humidity, create favorable conditions for disease development.

4. Identification:

  • Leaf Lesions: Identification involves recognizing the characteristic dark brown to black spots on macadamia leaves.
  • Nut Symptoms: When present, spotting on the nuts may further confirm the presence of Pseudocercospora macadamiae.
  • Microscopic Features: Microscopic examination may reveal the presence of conidia and other fungal structures.

5. Management:

  • Cultural Practices: Good orchard hygiene, including the removal and destruction of infected plant material, can help reduce the inoculum.
  • Fungicide Applications: Fungicides may be applied during periods of high disease pressure, following recommended guidelines.
  • Pruning: Proper pruning to enhance air circulation can reduce humidity levels and limit the spread of the fungus.
  • Resistant Varieties: If available, planting macadamia varieties with some level of resistance to the disease can be beneficial.

6. Importance:

  • Economic Impact: Leaf and nut spot caused by Pseudocercospora macadamiae can lead to reduced yields and affect the quality of macadamia nuts.
  • Global Distribution: The pathogen is of concern in regions where macadamia trees are cultivated.