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

Friday, July 5, 2019

Passionfruit - Hawaii’s Favorite Flavor




The yellow passionfruit, Passiflora edulis f. flavicarpa, and the purple passionfruit, Passiflora edulis, are grown in most tropical and subtropical parts of the world. The yellow passionfruit is a tropical plant while the purple is considered subtropical, being able to survive some freezing temperatures.


Of the hundreds of species in the genus Passiflora, these two, P. edulis, and P. edulis f. flavicarpa, are solely designated as passionfruit.  In Hawaiian, the fruit is called lilikoi and in Portuguese, maracuja peroba.  When the seeds of purple passionfruit first came to Hawai`i from Australia in 1880, they were planted in East Maui in the District of Lilikoi and that name stayed with the fruit. The seeds of the yellow passionfruit were brought to Hawai`i from Australia in 1923. 


In 1951, there were only a few acres of passionfruit plantings. It was then that the University of Hawai`i chose passionfruit as the most promising crop for development. And by 1958, there were 1,200 acres of primarily yellow passionfruit, and the industry was firmly established.

The lilikoi vine is a shallow rooted perennial, displaying beautiful, fragrant flowers, 2-3 inches wide. The fruit is nearly round approximately 1 ½ - 3 inches wide.  Inside, the fruit is filled with an aromatic mass of juicy pulp and within are as many as 250 small, edible seeds.  These vines, especially the yellow, are fast-growing and will begin to bear in 1 to 3 years. In fact, some vines can flower and fruit within a year after being started from seed. In Hawai`i, passionfruit matures from June through January; the ripe fruit will fall to the ground.

Carpenter bees are efficient pollinators for the yellow passionfruit. Honey bees and the hover fly also help in pollinating but are much less efficient. Wind is ineffective as a pollinator because of the heaviness and stickiness of the pollen.

The yellow lilikoi vine tends to be more vigorous and the fruit generally larger than the purple. While the purple appears to grow better at higher elevations, 400 to 3,000 feet, the yellow fruit is best adapted to lower elevations, from sea level to 1,500 feet. Furthermore, the yellow will yield 3 to 4 times that of the purple, yet the purple fruit is considered to have better flavor and aroma with the pulp being less acid with a higher proportion of juice.

Passionfruit vines are usually grown from seeds.  If the seeds are planted soon after being removed from mature fruit, most will germinate in 2 to 3 weeks. Fortunately, seeds do not require cleaning, drying or storage. They can be planted immediately after being removed from the fruit, even separation from the pulp is not necessary. In fact, allowing the pulp to ferment for a few days may hasten germination. In contrast, seeds that have been cleaned and stored actually have a lower and slower rate of germination. 

Propagation of passionfruit can also be accomplished through air layering and cuttings. Good soil drainage is essential for successful plantings.

Commercially, vines are trained on wire trellises. For backyard production, however, the yellow passionfruit is more productive and less subject to pests and diseases if allowed to climb a tall tree.

In Hawai`i, Oriental and melon fruitflies will deposit eggs in young fruit. This may cause fruit to shrivel and fall from the vine. If older fruit is pierced, the only ill effects will be an external scar. Other pests include aphids, scale, thrips and mites. In spite of all these, passionfruit or lillikoi vines flourish on fences and in trees, in backyards and vacant lots around the Islands.

The juice with its distinct flavor and aromatic bouquet is a key ingredient in making sauce, candy, ice cream, sherbet,  iced tea, or in cocktails.  In the Hawaiian Islands lillikoi is a favorite flavor enjoyed by young and old alike. 

Photos  by Forest and Kim Starr