Wednesday, December 4, 2019

Citrus Scab Disease


                                                       
What causes the scabby bumps on citrus fruit? This disorder, common in high rainfall areas, is called citrus scab. The disease occurs in Hawaii, as well as in Florida, but not in California due to the dry climate. It is caused by a fungus, Elsinoe fawcetti, and affects a wide range of citrus.

                                                       
The light brown, raised, warty scabs appear on young stems, leaves and fruit.  Leaves develop some resistance to citrus scab with age. The scabs consist of the body of the fungus plus the swollen tissue of the host plant.

Spores are produced within these scabs and will spread to other tissues by the splashing rain or irrigation water. Infection occurs when the spores germinate on a susceptible plant surface that remains wet for approximately 3 to 4 hours.
                                                 
                                                         

Control 
·        Select a resistant species or variety. 
·        Plant in a sunny and dry location as is available.
·        Intercrop citrus with non-citrus plants or resistant citrus trees.

Citrus scab can reduce yield and will certainly reduce the quality of the fruit. For home production, as bad as it looks, the fruit can still be sweet and juicy. For those caring about the external appearance of the fruit, trees can be sprayed with a copper fungicide. Yet even numerous applications may not be effective in high rainfall areas.

Highly susceptible citrus types: Fremont, Clementine, Murcott and Frost Satsuma mandarins; Orland Tangelo; Tahitian lime and Rangpur lime, often referred to as the common lemon in Hawaii.

Not susceptible or immune citrus types: Sweet orange, navel orange, pummelo and grapefruit.

Monday, November 4, 2019

Citrus - Remove First Year Fruit or Not?




This question comes up often in regards to young citrus trees. The answer is applicable to many fruit trees: trees spend energy in order to develop their fruit. For an older tree, there is usually enough energy to both grow the tree and develop the fruit. When a young tree produces its first bloom, there is a limited amount of energy. Will all the energy be used to grow leaves and branches or will some of it be applied to develop fruit? Many citrus growers will choose to sacrifice the first year’s crop - remove the young developing fruit - so their trees will grow quickly and eventually produce more fruit. 

On the other hand, if the first fruit is left until maturity, that fruit is using energy at the expense of the growth of leaves and branches.  For the homeowner, since the resulting differences in yields are generally small, my advice is to enjoy the first season and let the fruit set!

Monday, September 30, 2019

Problems with Acidic Soils: Lime Can Work Wonders




Note: pH is the measurement of the soils acidity/alkalinity. The pH scale is from 0 to 14 with 7 as the neutral point; below 7 is acidic, above 7 is alkaline. Here are a few common pH examples:

  • Extremely acid:  lemon-2.5; vinegar-3.0; stomach acid-2.0; soda pop-2–4

  • Strongly acid:  carrots-5.0; asparagus-5.5; cabbage-5.3

  • Slightly acid:  cow's milk-6.5

  •  Neutral:  saliva-6.6–7.3; blood-7.3

  • Slightly alkaline: eggs-7.6–7.8

  • Moderately alkaline: sea water-8.2; sodium bicarbonate-8.4

  • Very strongly alkaline: soapy water-12; household bleach-12.5


Concerning the pH of soils, there is a preferred range in which plants will grow best. Most soils in Hawaii, as in other parts of the country, are acidic. Extremely acid soils can have a toxic effect on many plants. One common problem is excessive aluminum and manganese becoming soluble in the soil adversely effecting plants.  Acidic conditions can also change the type of microorganisms living in the soil. This will affect the amounts of nitrogen, sulfur and phosphorus available to the plants since the microorganisms breakdown these nutrients into plant-available forms.

These acidic soils will benefit when lime is applied. As an amendment, lime in its various forms can raise the pH of the soil and eliminate a number of problems.
Ground limestone is almost pure calcium carbonate and comprises the largest percent of lime used in the United States. It is usually the cheapest form of lime. Limestone with significant amounts of magnesium carbonate is called dolomitic limestone (dolomite), containing approximately equal parts of magnesium and calcium carbonate. Dolomite works well especially if the soil has a magnesium deficiency.

Hydrated lime, another form, is one and a half times more effective at raising the soil pH but is also more caustic, and therefore,  hazardous to handle. Protection for the skin and lungs is recommended. Excessive amounts of hydrated lime can cause a pH shift towards a highly alkaline condition. Conversely, when using dolomitic lime, it is difficult to apply too much as long as it is thoroughly mixed into the soil.

When purchasing lime, a fine powdered lime, about 60-100 mesh, is the fastest acting grade available.  However, being a fine powder, it is a potential eye irritant under windy conditions. It will also cake rather easily, and when improperly mixed, it will stratify forming a layer that can burn roots and repel water.

A coarser limestone (similar to salt grains) is also available and is easier to handle, but its disadvantage is that it is slower acting.  In summary, incorporating lime as an amendment in any of its forms can make a big difference in acidic soils.

Friday, July 26, 2019

The Plague of Powdery Mildew





Powdery mildew (PM) is a menace to many gardeners. The disease will turn large zucchini leaves white and later brown. A whitish cast will appear on many common garden plants such as tomatoes, beans, cucumbers, mustard, peas, and collards. Infected leaves often turn yellow to brown and may shrivel making the plant unproductive and eventually die.


The white fungal growth can develop both on the upper and the lower surfaces of the leaf, and sometimes, on flowers and fruit. The cottony like threads of this fungus travel along the surface of the leaf, occasionally sending “roots” down into the leaf tissue in order to obtain nutrients.


Many different types of powdery mildew fungi are host specific. This means that one particular mildew fungus will only infect those plants in a particular genus or family; the PM on the beans will not attack papaya, and the PM on mangoes will not attack tomatoes. 

The fungus likes temperatures between 60 and 80 degrees F and is sensitive to temperatures above 90. Cool days and warm nights favor the disease. Spores of the pathogen are dispersed readily by wind. Even though this disease flourishes with high humidity, wet leaves can actually inhibit germination of the fungal spores, thus preventing infection. Because of this, the disease should be minimized during periods of heavy rainfall. Also during this time, the numerous spores that are on the leaves will be washed away.

Control Stopping powdery mildew in its earliest stages of development provides the best control. This can be done by sanitation: remove and destroy infected parts of the plant. Other measures of control are as follows: plant in the sunniest locations, provide good air circulation through pruning, and avoid excessive applications of fertilizer.

Fortunately there are a number of relatively safe and effective materials to use against this fungus - wettable sulfur, horticultural oils, including neem, and Kaligreen (potassium bicarbonate a relative of baking soda). Serenade is a bacterium and is considered a biological fungicide. It helps to prevent the powdery mildew from infecting the plant. Beware: Cucurbits (melon, squash, cucumbers) can be sensitive to sulfur.  Do not apply when the temperature is near or over 90 degrees and do not apply within 2 weeks of an oil spray. Use these materials in the earliest stages of disease development for best results.  

Before spraying, it would help to remove the leaves that are heavily infested.  Do not dispose of them on the ground since they are loaded with fungal spores. Powdery mildews can attack healthy plants; but older plants, less vigorous plants and those that are stressed are more susceptible to infection.  
Most important, choose plant varieties (vegetables, fruit trees and ornamentals) which are tolerant or resistant to the powdery mildew fungus. For more information about seeds from locally developed vegetable varieties, contact the University of Hawai‘i at Manoa: http://www.ctahr.hawaii.edu/seed/seeds.asp. Or, contact your local university cooperative extension office. 

Note: Some of the resistant varieties will exhibit powdery mildew symptoms, but the disease is less severe. 

Photos: Cornell University, Colorado State University and USDA