Nutrient Management Research Database

This work aimed to determine the relationship between tree nitrogen status and three fungal diseases, brown rot blossom and twig blight, shot hole and scab, and hull rot disease. Brown rot, shot hole, and scab are usually effectively controlled by fungicide applications, whereas a control is not available or likely for hull rot. Cultural practices in general, and nitrogen fertilization in particular, are known to affect plant disease incidence and severity. Demonstration that nitrogen levels affect almond diseases should encourage growers to include evaluation of their nitrogen practices when designing fungicide programs for disease control. Even through reduction of nitrogen alone may not produce acceptable control of plant diseases, conditions that render plants more susceptible to disease, e.g. nitrogen fertilization, could negatively affect efficacy of fungicide treatments. Diseases not controlled by fungicides, such as hull rot, in pat may be managed by changed in nitrogen fertilization. Results of this work should that excess nitrogen use can exacerbate disease problems and may compromise effectiveness of fungicides. These data should be useful in educational efforts directed at improved disease control, reduced nitrogen use, or both.

• Researchers used the appropriate pathogens to inoculate flowers with brown rot, young fruit with green fruit rot, and maturing, dehiscent fruit with hull rot.
• They then measured and evaluated the responses to disease of the plants.
• Four nitrogen treatments were used, 500, 250, 125, and 0 lbs of nitrogen per acre, 2/3 of which was applied in the fall, 1/3 applied in the spring.
• The trees were treated once annually with a fungicide at bloom.
• The study was conducted for four years.

• Results show that with each disease, disease incidence increased as nitrogen treatments increased, although levels of disease did not vary greatly between the 250 and 500 lb of nitrogen treatments.
• No evidence of a threshold of leaf nitrogen content that would signal an upsurge in disease was found.
• There are many factors that lead to disease susceptibility in trees, but this work does suggest that:
  • growers should take care not to exceed optimum nitrogen levels
  • growers should consider reducing nitrogen application to control disease especially when growers may have serious disease problems or pressures.