Monday, June 29, 2020

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 being 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 biological fungicide, a bacterium, that helps prevent the powdery mildew from infecting the plant. 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

Saturday, May 30, 2020

Plant a Rainbow of Colors in Your Garden


When planting a vegetable garden, let color be your guide to healthy eating.  For a number of years, scientists have been discovering the health benefits of the color pigments in food.  For some gardeners, a vegetable garden is lettuce, collard, mustard greens and kale- all healthy, leafy green vegetables.   But for health sake, add some color such as red tomatoes, yellow squash and purple eggplant.  Grow a rainbow of colors to the garden for good health.

Red colored fruits and vegetables contain natural plant pigments called lycopenes or anthocyanins. These compounds may help reduce the risk of several types of cancer, especially prostate cancer. By the way, the lycopenes in cooked tomatoes with a small amount of fat are absorbed better than lycopene from raw tomatoes.  Sun dried tomatoes are reported to have twelve times the lycopenes as raw ones. So plant plenty of tomatoes, beets, watermelon and red peppers. If there is room in the garden, include a pigmented citrus tree like pink grapefruit. 

Orange and yellow colors come from natural plant pigments called carotenoids.  Beta-carotene is converted to vitamin A which helps maintain healthy eyes.  Carotenoid-rich foods can reduce the risk of cancer, heart disease and improve the immune system function.  Plant plenty of yellow sweet potatoes, squash, pumpkins and carrots.  Citrus contains hesperidin, also found in the skin of tomatoes, and naringenin, which are anti-inflammatory and antioxidant. Curcumin, found in turmeric, known in Hawaiian as olena, has antioxidant properties. Turmeric is a popular Indian spice used in curries and other dishes.  Anecdotal evidence suggests that turmeric can be used to deter ants in the garden.

Other yellow and orange fruits grown in Hawai`i are pineapples, papayas and mangos. Pineapples contain bromelain, an enzyme which aids digestion.  Papaya and tangerines contain beta-cryptoxanthin, another carotenoid, playing an important role in vision and in bone growth. Papayas can easily be grown from seeds or purchased in abundance at local markets.

 Greens are colored by a natural plant pigment called chlorophyll.  Some greens contain lutein which helps keep eyes healthy. Here is a familiar list of green vegetables: green peppers, peas, parsley, watercress, arugula, spinach and kale.  Crucifers like broccoli and cabbage contain chemicals which may help protect against some types of cancer.  Leafy greens such as spinach and broccoli are excellent sources of folate, which is a B vitamin. 

The blue and purple colored fruits and vegetables also contain pigments called anthocyanins, powerful anti-oxidants which improve brain function and help to reduce the risk of cancer, stroke and heart disease.  Fig trees grow well in tropical and sub-tropical climates and should yield delicious, healthy fruit.  Anthocyanins, also found in strawberries and raspberries, act as powerful antioxidants that protect cells from damage. 

The whites contain pigments called anthoxanthins which are shades of white or yellow.   This group consists of onions, garlic, cauliflower, turnips, mushrooms, potatoes and bananas.  At least one of the group, garlic, contains a health-promoting chemical called allicin. This compound may help lower cholesterol and blood pressure and help reduce the risk of stomach cancer and heart disease. Potatoes, as well as bananas, are good sources of potassium. 


Sunday, May 10, 2020

Raised Bed Gardening - Pros and Cons




Many people are drawn to raised bed gardening for several reasons.  In addition to the fact some areas do not have sufficient soil, raised bed gardens generally look nice and appear easy to maintain. In many parts of the United States, reasons for building raised beds are to keep gophers out or to have warmer soils in the early spring after the winter thaw. For portions of the Hawaiian Islands, gardening in raised beds or containers is necessary because of a lack of soil on lava hardened  terrain.  

Other reasons for building raised beds are 
 (1.) the soil may be extremely alkaline (a high pH) or acidic (low pH), 
 (2.) the soil is too compacted,  
 (3.)  the gardener needs wheelchair access and
 (4.)  the gardener prefers less stooping.

On the other hand something to consider is the maintenance of raised beds. If the frame is made of wood, it will eventually rot. If metal is used, it may rust. Actual construction of the bed can be time consuming and costly. In addition to wood, rock and plastic can also be used.

Two questions often arise. First, how deep to make the beds?  The literature gives a range of 6” to 18”.  Crops like lettuce, onion and bok choy will do fine in a shallow bed. Yet deeper beds are needed for carrots, parsnips, corn and tomatoes.  

Secondly, where to get good soil? You can buy bagged potting mix at the garden store, which can be very expensive. Or you can buy a truck load of soil but be cautious. Soil may look good, but you can’t see N-P-K deficiencies or excesses, pathogenic fungi, bacteria, nematodes or pesticide residues including persistent herbicides. Bringing in soil can be risky.


Installing raised beds can be a satisfying project. But first, find out what your particular problem is and why you are considering a raised bed. Perhaps correcting the problem could be a better solution, possibly cheaper and less time consuming.

Photos from University of Missouri.

Monday, March 30, 2020

Why fruit trees don’t produce.


Since there are many factors involved in fruit production, there are also many reasons why fruit trees don’t produce.  Here are a few of the more common ones.
·        Rain- Too much rain during the bloom period can wash pollen away,  and a forceful rain along with wind can actually knock blossoms off.  Also, bees tend to fly less, or not at all, during heavy rains.
·        High heat and low humidity during bloom will hamper pollination.
·        Juvenility  - Sometimes it takes many years before a tree will produce fruit; mangosteen is a good example. However, new evidence indicates the size of the tree is more of a factor than age. In general, trees grown from seed take longer to produce fruit than those that are grafted, budded, or air layered. 
·        Pollinators – Some fruit trees require more than one tree or one variety in order to set more abundant fruit.  Examples are rambutan, macadamia, and avocados.
·        Some trees require insects to help in pollination, such as durian. 
·        Lychee and longan need a combination of low temperatures, 50 -59 degrees Fahrenheit, and a dry period (4-6 weeks) before bloom occurs.  Bloom for rambutan will be enhanced in response to drought.
·        The deciduous fruit trees, like peach, plum, nectarine, apple, etc., require cooler temperatures for good fruit production. This chilling requirement is the number of hours the tree needs to experience below 45 degrees F.  This requirement differs among species and varieties, ranging from a few hundred hours to over a thousand. 

Monday, February 3, 2020

Cilantro or Coriander?





Most chefs are familiar with the seasoning coriander. Coriander is actually the small, dried fruit, often referred to as the seeds of the cilantro plant. In fact, the scientific name for cilantro is coriandrum sativum, or coriander, also known as Chinese parsley. The fresh leaves and the small dried fruit are the parts most traditionally used in cooking.

The fresh leaves are often used in South Asian and Chinese cooking as well as in Mexican cooking, particularly in salsa and guacamole.  Since heat will diminish the flavor, leaves are frequently used raw or added just before serving. The leaves lose their aroma when dried or frozen.

Coriander seed is a main ingredient in garam masala, an Indian spice, and in curries. Although the ingredients in garam masala can vary, it commonly includes coriander, black pepper, cumin, cardamom and cinnamon.  In India, roasted coriander seeds, called dhana dal, are eaten as a snack.

Coriander is used for pickling vegetables and for making sausages in Germany and South Africa. In Russia and Central Europe, the seeds are used as an alternative to caraway seeds. Even in brewing certain types of beer, particularly some Belgian wheat beers, coriander is an added flavor.

The Problem of Bolting


Cilantro is a cool season plant. It flourishes during cool nights and moderate, sunny days as in the spring and fall. But it will bolt at the first sign of hot weather. Bolting is the premature flower formation initiated by hormones within the plant system in response to high temperatures, as well as drought and starvation.  This unwelcome occurrence in leafy vegetables such as cabbage, lettuce and cilantro, takes the plant out of its leaf producing mode and switches over to flower and seed production.  

On the Big Island of Hawaii, the Volcano and other similar areas would perhaps give cilantro the longest season, provided there is adequate sunshine. Cilantro, however, is a true annual and even under the best conditions, it will send up flowers and eventually die within the year. In order to minimize the bolting effect, cilantro should not be grown during the warmer, summer months. The best time for planting would be after the summer heat from September or October until perhaps March.

There are some slow-bolting varieties, Calypso is one, that can extend cilantro’s productivity perhaps a month. Snipping off the first newly emerging flower buds may also extend the leafy period, but only slightly. 
 
Like many plants, coriander may contain properties useful in the treatment of Type 2 diabetes and high cholesterol.  Research is presently investigating these claims and other medicinal attributes of coriander.