Barley will be more susceptible to disease and pests where nitrogen and potassium availability are imbalanced. This imbalance can result in weaker crop growth, containing a higher concentration of soluble nitrogen compounds and carbohydrates. The latter provide a readily available food source for obligate parasite diseases such as the rusts, powdery and downy mildew. Excess nitrogen can increase disease incidence. With the facultative pathogens e.g. Altanaria and Fusariums, nitrogen will decrease disease incidence.
Crop Nutrition and Barley Health
It is also important to maintain a balance between nitrogen forms present i.e. nitrate and ammonium as both of these have been identified at reducing specific plant diseases (nitrate reduces diseases from the Fusarium, Botrytis, Rhizoctonia and Pythium species, whilst ammonium has decreased severity of diseases from the Pyricularia, Gibberella and Sclerotium species).
When phosphate is limiting, barley yield reduction and increased susceptibility to disease can occur. This response is again related to phosphorus deficiency restricted plant metabolism and growth leaving the developing plant prone to pathogen attack. Research has shown the increased incidence of mildew when phosphate is not applied by 50%.
Potassium deficient plants are poor at synthesising the products such as starch, proteins and cellulose giving rise to an accumulation of smaller compounds such as amides that are the food for diseases. Thinner cell walls with less mechanical resistance to predators may also result from potassium shortage. A review of over 1000 cereal trials found that where potash levels were low and out of balance with nitrogen supply, application of potash reduced disease and bacterial infections in over 70% of cases (PDA) in barley.
Potassium deficiency has also been linked to higher rates of rust infection and powdery mildew incidence.
Manganese has been associated with disease control in numerous investigations. It has a direct inhibitory effect on fungi growth, especially powdery mildew, as well as being involved in lignin and suberin production giving plant cells more physical resistance to infection. Manganese deficient plants have a poor ability to metabolise nitrogen giving an accumulation of nitrate nitrogen in the leaf. This acts as a food source for the diseases such as rusts and mildew, increasing the levels of infection in barley.