EPWS 310 - PLANT PATHOLOGY
NEMATODES - PARASITIC HIGHER PLANTS - PROTOZOA
Nematodes, also called eelworms, are free living soilborne organisms. They survive in the soil and feed on bacteria, microscopic plants and animals. Numerous species attack and parasitize humans and animals.
- Well known human pathogens - 'pinworms', Elephantiasis, and trichinosis.
- They live in water in soil pores and the narrow layer of water on soil particles. The plant pathogens are the biggest problem in sandy soils.
Morphology - 300-1000 m m, with some up to 4 mm long. Easily observed under a microscope. Round in cross section with smooth, unsegmented bodies, without legs or other appendages. The females of some species, become swollen at maturity and have pear-shaped or spheroid bodies. (figures 15-2, 15-3 ).
Anatomy- Transparent covered by a colorless cuticle. The cuticle molts when a nematode goes through its successive larval stages. The digestive system is a hollow tube extending from the mouth through the esophagus, intestine, rectum, and anus. Lips, usually six in number, surround the mouth.
-All plant parasitic nematodes have a hollow stylet or spear, which is used to puncture plant cells.
-Two feeding habits: Ectoparasites - feeding from the surface of plants.
Endoparasites - penetrate the plant tissue.
-The hollow stylet allows the nematode to pierce plant cells and ingest their contents.
Reproduction- the female has one or two ovaries, followed by and oviduct and uterus terminating in a vulva. The male has a testis, seminal vesicle, and a terminus in a common opening with the intestine. A pair of protrusible, copulatory spicules are also present in the male. Reproduction in nematodes is through eggs and may be sexual, hermaphroditic, or parthenogenetic. Many species lack males.
*Hermaphrodites- have male and female sex organs.
*Parthenogenetic- capable of reproducing without males.
Life cycle- they begin with as an egg and mature into an adult after four molts. In between, the nematodes exist as "juveniles" of increasing size. In most cases these juveniles look the same as the adults but smaller. Adults are sexually mature. Plant parasitic nematodes are obligate parasites. Second stage juveniles hatch from eggs and move to host plants to feed.
NOTE - many nematodes interact with other pathogens. E.g. root knot with Fusarium oxysporum
Some economically important nematodes-
1. Seed gall nematodes - Anguina sp.
These were the first nematodes discovered to destroy plants. These nematodes swim from the soil to the developing grain head in a layer of water on the plant surface and burrow into the developing flowers. They form galls in the fruit. These galls were often replanted the next year.
2. Pinewood nematode - Bursaphelenchus sp.
This nematode infects and kills pine trees. Pines in the U. S. are somewhat resistant- e.g. Norway or blue spruce. However, Scotch pine is highly susceptible. Japanese forest of red and black pines have been totally wiped out by this nematode. Restrictions on U.S. lumber to Europe and Asia have resulted because of the pine wood nematode.
3. Lesion nematode - Pratylenchus sp.
Also called the pin nematode of many plants. This nematode penetrates and feeds on root cells, leaving behind dead and dying cells that become brown. The lesions are often invaded by other microorganisms. Endoparasite.
4. Cyst nematode - Heterodera sp.
Big problem on Soybean and cereals. It is often called the Soybean cyst nematode-H. glycines. This nematode overwinters as a brown cyst that is formed on the roots.
5. OVERHEAD FIG 15-11 - LIFE CYCLE
**Root knot nematode - Meloidogyne (several species). Most important plant pathogens: M. incognita, M. arenaria, and M. javanica.
More than a dozen species known affecting over 2000 spp of plants. The root rot nematode is distributed throughout the world and occurs in a wide variety of crop plants. It is most severe in areas with warm climate, and causes extensive economic loss in nursery plants as well as in row crops.
Symptoms- Primary symptom is tumefaction- irregularly shaped galls. Apical growth ceases. Lesions sometimes are present. Feeding by both the males and females stimulates hyperplasia and hypertrophy in the roots.
Overwinters- eggs between crop seasons in the soil. In a warm, moist environment the eggs hatch, liberating the second larval stage.
Colonization- Male nematodes live parasitically in the roots for several weeks and then undergo three molts in rapid succession before emerging from the root. Adult males are thought to live free in the soil. Males are rarely found in the most important Meloidogyne pathogenic species.
Females that have entered the roots remain there after molting and increase in size. The become pear shaped about 3 weeks after penetration, and they protrude into what would have been vascular tissue. Cell walls in the vicinity of the head of sedentary females are digested and the several adjoining cells coalesce to form 'giant cells'. Food materials essential to the continued nourishment of the nematode collect in these cavities.
At maturity, the posterior end of the female either protrudes through the surface of the gall tissue or lies very near the surface. Eggs are laid in a gelatinous matrix extruded from the vulva.
Control - Nematicides and fumigation
- Biological control- Bacillus sp.
- Mycorrhizal fungi - Glomus sp.
PLANT DISEASES CAUSED BY PARASITIC HIGHER PLANTS
- 2500 species of higher plants are known to live parasitically on other plants.
-They vary greatly in their taxonomic botanical classification.
-Some have chlorophyll, and some have other pigments
-Some attack roots, others attack shoots.
-Only a few of the known parasitic higher plants cause important diseases on agricultural crops or forest trees.
Botanical families and genera of important parasitic higher plants
I. Those that attack above ground portions of plant.
Genus: Cuscuta, the dodders
Genus: Arceuthobium, dwarf mistletoe of conifers
Genus: Phoradendron, the American true mistletoe of broad-leaved trees.
Genus: Viscum, the European true mistletoes
II. Those that attack below ground portions of plant.
Genus: Orobanche, the broomrapes of tobacco
Genus: Striga, the witchweeds of many monocotyledonous plants.
1. Dodder-Cuscuta sp.
Occurs in Europe and North America.
Host: Problem on alfalfa and clover. Other crops, onions, sugar beets, ornamentals, and potatoes.
Symptoms: Orange or yellow vine strands grow and entwine around the stems and the other above ground parts of the plant. Produce white, pink or yellow flowers. The infected plant becomes weakened by the parasite; vigor decreases and yield declines.
NOTE DISEASE CYCLE (Fig. 13-2)
Prevent introduction - exclusion
-Limit the movement of domestic animals (sticky seed capsules)
-Spray with contact herbicides. 2,4-D
- Spot spray at low concentrations with Glyphosate (Round-up) has been shown in some studies to be effective even when it was applied after dodder was attached to alfalfa and controlled the pathogen without causing apparent injury to alfalfa.
2. Dwarf mistletoe of conifers-
Occurs in all parts of the world where conifers occur. Big problem in Southwest and Pacific coast. Trees can be retarded, deformed or killed.
Growth height reduced by 50 to 80%. Timber quality reduced.
NOTE DISEASE CYCLE (Fig. 13-7)
-Plants either male or female - dioecious
- Produce flower after 4-6 years. Male dies after flowering. Female dies after fruit dispersal.
-Internal pressure expels the seed upward or obliquely (out) for distances up to 15 meters.
-Birds can also transmit seeds.
Control: remove diseased part of tree.
-Resistance is being sought.
PLANT DISEASES CAUSED BY FLAGELLATED PROTOZOA
Small single celled flagellated animals related to the sleeping sickness pathogen (Trypanosoma) transmitted by Tsetse flies
Thought to cause 6 - 10 diseases of tropical plants including:
Phloem Necrosis of Coffee
Hartrot of Coconut Palms
Caused by species in the genus Phytomonas
No vectors known: no means of control except sanitation and quarantine
END OF THE PATHOGENS!