By Russell T. Nagata
University of Hawaii at Manoa College of Tropical Agriculture and Human Resources Komohana Research and Extension Center, Hilo
Parasitism in the garden is a common occurrence; although most gardeners are unaware it happens and it generally occurs unobserved.
Most common observable parasitism is that of aphids and caterpillars by small wasp. Parasitized aphids transform into round, tan colored mummies from which adult wasp emerge after utilizing the aphid’s body as room and board. Live caterpillars are transformed into a nursery for hundreds of larval wasp, which upon pupating emerge to cover the caterpillar’s body with tiny white silk pupa cases.
Less commonly observed in our gardens, is the parasitism of plants by other plants. A living plant that takes nutrients from another living plant is called a parasite. The plant from which nutrients are taken is called the host. The degree of parasitism of a host will vary by the dependence of the parasite on the host.
Hemiparasitic plants are capable of photosynthesis and only require water and nutrients from the host. Mistletoe is a good example.
Holoparasitic plants are completely dependent on their host plant for all of their growth needs. One example is the dodder. The host/parasite relationship rarely causes the death of either and is thought to be an unhappy balance.
Parasitic plants are sometime labeled “vampire plants,” since they derive nourishment from their host. The means by which parasitic plants extract nutrients and water from the host plant is through direct connection of the respective vascular systems. A small specialized appendage call a haustorium taps into the host vascular system.
After seed germination and the start of plant growth, the parasite recognizes a host plants by chemical stimuli and responds by producing a haustorium which attaches to the host plant.
From the haustorium, a penetration peg grows into the host tissue and establishes a vascular connection. Failure to find a host plant will result in death of the parasite seedling. In Hawaii there are six known species of mistletoe (Korthalsella spp.) and are known as kaumahana or hulumoa. While they don’t resemble mistletoe depicted with Christmas and Druid celebrations, they do have similarities. The Hawaiian mistletoe grow on branches of large native hardwood trees and most often seen in forested areas. Hawaiian mistletoes have much reduced leaf size and in many cases enlarged, flatten stems and are said to resemble cacti.
Common host plants are the ohia, koa, akolea, and lonomea. Several parasitic vines can be found growing in Hawaiian gardens. Cassytha filiformis, the laurel dodder, kauna‘oa, pehu, kauna‘oa malo-lo, kauna‘oa uka, kauna‘oa, malolo, pololo, love vine, or woe vine, is a leafless vine green-to-orange in color. It parasitizes many woody trees and shrubs growing in Hawaii and is a perennial plant. In many locations, Cassytha are found growing in the crown of trees.
The dodder, Cuscuta sandwichiana, kauna‘oa or kauna‘oa-lei, is a native orange, stringy appearing vine normally found growing near the ground around the state. Its host plants are mainly herbaceous annuals upon which it is completely dependent on. It also represents the island of Lanai as the island lei. In Hawaiian legend, this dodder is known as the motherless plant because it is a parasite which grows on another plant.
A closely related species, Cuscuta campestris, appears to be similar in appearance, however is a non-native from north America. Cassytha and Cuscuta can be differentiated in the following manner. Flowers of Cassytha is borne in small panicles or clusters and have fleshy seeds, while Cuscuta flowers are borne singly and the fruits are dry.
The sandalwood, Santalum spp., while not a common garden plant, can be found growing in a number of gardens. Four species of sandlewood are recognized in Hawaii. They are ‘iliahialo‘e, S. ellipticum, ‘iliahi, ‘a‘ahi, ‘aoa, la‘au ‘ala, wahie ‘ala, S. freycinetianum, S. haleakalae, and S. paniculatum. All are root parasites in that the connection to its host is through root contact. Currently recognized host plants are koa, aheahea and Chamaesyce hypericifolia. Native grasses are also thought to be host plants.
Parasitic green alga is fairly common, especially on the windward side, but is often overlooked. The parasite is a filamentous green alga, either Cephaleuros virescens or Cephaleuros parasiticus. You can find C. parasiticus as spots on guava leaves and fruits and is a parasite that infects right through the leaf and causes subsequent leaf death in the infection area.
C. virescens can appear on the upper leaf surface of hundreds of plants as circular flattened mounds, burnt-orange to brown or rust-colored, up to about 2 centimeters in diameter. A good place to look is on your avocado tree. Other important plants infested by parasitic alga are tea, coffee, coconut, cacao, breadfruit, magnolia and mango, to name a few.
For more information on this and other gardening topics, please visit the CTAHR electronic publication website at http://www.ctahr.hawaii.edu/Site/Info.aspx or visit any of the local Cooperative Extension Service offices around the island. I can be reached at firstname.lastname@example.org.