Black Streak of Banana

 What causes the yellowing and streaking in banana leaves? 

 Banana plants are commonly infected by a fungal disease called Black Leaf Streak of Banana.  The leaves exhibit conspicuous yellowing, but upon closer examination you will find dark spots which enlarge to form the characteristic streaking effect. The streaks may vary in color. 

The principle means of spreading the disease is rain, which splashes the fungal spores to new leaves.  Fruit quality and yields can be reduced due to the disease.  It is unlikely that the banana tree will ever be completely free of the disease; however, pruning and disposing of infected leaves will keep the trees productive. Make sure the tree has adequate water and fertilizer to stimulate new growth; this will keep the tree one step ahead of the disease.  Also, removing some of the sucker growth will increase air flow in the canopy and reduce humidity, both of which increase severity of the disease.  Of the commonly planted commercial varieties, the dwarf Brazilian is less affected by black streak.

What does N-P-K mean?

What do the three numbers on a fertilizer package mean?   

All fertilizer packages have three numbers printed on them; for example, 10-15-5. These numbers represent the percentage of the three primary plant nutrients, nitrogen, phosphorus and potassium.  In the above example, the fertilizer would contain 10% nitrogen, 15 % phosphorus and 5% potassium.  This is important to know because different plants and different situations require different types of fertilizer. 

·        Nitrogen is important in the photosynthesis process and for many growth and developmental processes.  It moves readily in the soil and roots have easy access to it.  But during periods of heavy rainfall, the nitrogen is leach beyond the root zone and can contribute to groundwater pollution.  Deficiency symptoms include stunted growth and chlorotic (yellow) and pale green leaves.  These symptoms usually begin in older tissue because nitrogen is mobile within plants and moves from older to younger tissue when in short supply.  Plants with excess nitrogen will exhibit dark green leaves, reduced yields and delayed maturity.

·        Phosphorus stimulates early growth and root development and promotes the formation of seeds.  It is also important for the energy systems of the plant.  Soil phosphorus is often tied up chemically in relatively insoluble compounds and therefore does not leach as much as nitrogen will.  Phosphorus fertilizer uptake by the plant is more efficient when in the presence of nitrogen.  Although some Hawaiian soils have adequate levels of phosphorus, many are low in this element.  Deficiency symptoms include slow growth, dark green leaves and a purplish color on older leaves.  Excess phosphorus can interfere with the uptake of micronutrients.

·        Potassium is involved with cell division, starch and protein synthesis and sugar movement within the plant.  It increases size and quality of fruits and vegetables and increases disease resistance.  Potassium is usually abundant in soils, but much of it is tied up in soil minerals and is unavailable to plants.  Soils in high rainfall areas are usually low in potassium. Deficiency symptoms include slow growth, small fruits and chlorosis on the leaf tip and margins.

You can buy fertilizers containing a single nutrient, like ammonium nitrate and urea, containing 33% and 45% nitrogen, respectively. Or, one containing a double nutrient like ammonium phosphate, which is 11% nitrogen and 48% phosphorus.  You can also purchase what is called a complete fertilizer which contains all three primary nutrients.  The term complete should not be interpreted to mean that the fertilizer supplies all the nutritional needs of the plant.

Many plants have similar nutritional requirement, but many have differing requirements.  A lawn, which is grown for its foliage and not fruit or flowers, will basically need nitrogen applications, with P and K added only occasionally, depending on their soil content.  Bananas require ample potassium; a fertilizer similar to 10-5-20 would be appropriate.  Papayas do well with a more balanced fertilizer. Vegetables in general like a high phosphorus content, like 10-30-10.  In determining which fertilizer to use, it is always best to have the soil analyzed; at least the primary nutrients, N-P-K.

Grapefruit Melanose

What causes the brown speckling on the skin of grapefruit?  The staining is a fungal disease known as melanose. This disease displays a very interesting pattern on the fruit called ‘tear staining.’ It is caused when dew, contaminated with spores, runs down over the surface of the fruit. As this happens, the fungal spores can stick to the skin of the fruit. These spores germinate and cause tiny spots on the fruit in the pattern of the water running down the fruit, hence the name ‘tear staining’. 

Unless the disease becomes very severe, melanose is considered a cosmetic problem and, in commercial operations, will cause a downgrading of the fresh fruit. 

For the homeowner, the fruit is just as good inside as any perfect looking piece of fruit at the supermarket.

With an abundance of rain, the spores splash to nearby leaves and twigs, causing leaf drop and twig dieback.  Many of the dead twigs we see in citrus trees are caused by this fungus.  (Note: In high numbers, scale insects can also cause twig dieback.)

Although a tedious job, it is best to prune out all the dead twigs, they are store houses of the fungal spores.  However, whether you prune or not, the trees will not die from melanose.