Plant diseases are caused by fungi, bacteria and viruses.
Most plant diseases are caused by fungal spores that tend to become a problem, especially during the wet weather season.

Fungi
Fungi are unable to generate nutrients on their own and in order to survive must derive their food from other organisms. Fungi attack all parts of a plant and under friendly conditions, fungi can damage plant translocation tissues; killing a plant in a quite short period of time. Some of the most common fungal diseases contain damping off, leaf spot, anthracnose and rust.

Bacteria
Plant diseases caused by bacteria are not as common as those caused by fungi. Damage caused by bacterial infection consequences primarily in rotting of the plant tissue.

Viruses
Viruses in plants are transmitted mainly often by sucking insects such as aphids and whiteflies. Viral diseases in general cause less damage than those vectored by fungi and bacteria. Infected plants typically become partly injured and weakened, however they infrequently die.

Plant diseases can spread without problems and rapidly. It is easier to fight and control fungal diseases than to eradicate bacteria and viruses. The best technique to prevent diseases is to take a proactive approach by preventing environmental factors that contribute to diseases combined with a healthy regimen that provides optimum health for plants.

In most cases, diseases can be prevented by using proper cultural practices such as variety selections, irrigation and humidity management, plant and soil nutrition, pruning, and row spacing. When there is insufficient circulation of air, poor water drainage, very expensive irrigation, and too much dampness due to rainfall, the fungi can become a problem.
The organic solution to these problems is to utilize copper and sulfur products. Spores are prevented from growing because the crop has been covered with the appropriate physical barrier.

It is essential to make the applications prior to rainfall to maximize success.
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Nitrogen (N) requirements increase rapidly with potatoes plants growth.
When N is translocated to upper leaves excessively from the lower leaves, they then become yellow.

Later, if the deficiency is not corrected by fertilization, the entire potato plant becomes yellow and fails to grow properly.

Severity of plant response depends on the level of N deficiency. N toxicity from ammonium or nitrites may follow degradation of nitrogencontaining fertilizers in certain soil conditions.

Phosphorus (P) deficiency follows P fixation in a wide range of soil types.

Symptoms include retardation in growth of terminals; small, spindly, somewhat rigid plants with crinkled or cup-shaped leaves; darker than normal colour; possibly a delay in maturity; and reduced yield.

Potatoes tubers may have internal rusty brown necrotic flecks similar to internal heat necrosis.
Because P is frequently fixed in the soil, fertilizer banding applications lateral to the seed piece are superior to broadcasting.
Potassium (K) deficiency is common in light, easily leached soils.

Early symptoms are dark or bluish green glossy foliage.
Later, older leaves of potatoes plants become bronzed and necrotic (superficially resembling early blight), and senesce early.
Necrotic, somewhat sunken corky lesions form on the tuber surface, particularly at the stolon attachment.
Potatoes tubers are predisposed to black spot, and when cooked tend to darken.

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