Pathogens for pest control
Resistance of insects to pesticides is becoming an issue of
increasing importance around the world. One particularly severe pest,
which is found throughout tropical, sub-tropical and temperate
regions, is the diamondback moth, Plutella xylostella. This
insect is a pest of crucifers, particularly brassicas such as cabbage.
Due to the excessive use of insecticides (spraying 2-3 times a week),
this insect is now resistant, in some regions, to all known categories
of insecticides including the bacterial biotoxin Bacillus
thuringiensis (Bt). A solution may now lie in the use of fungal
pathogens.
Scientists at IACR-Rothamsted (Institute of Arable Crops Research)
based in the UK are developing novel strategies combining
semiochemicals and entomopathogenic fungi for diamondback control. One
species of fungus, Zoophthora radicans (Entomophthorales),
naturally regulates diamondback populations in the field. Commonly,
however, it only reaches epidemic proportions too late in the season
to prevent crop damage. Rothamsted's idea is to encourage the
initiation of epidemics earlier in the season in small pest
populations, to maintain damage below the economic threshold.
To achieve this, traps have been designed and baited with synthetic
female sex pheromone to lure male moths inside. Once inside the trap,
the male moths become contaminated with sufficient fungal inoculum to
guarantee that they become infected. Once habituated to the pheromone,
the male moths leave the trap and return to the crop where they will
die and produce more infective fungal spores. Each dead male, which
remains attached to the plant, then acts as focus of infection that
rapidly spreads to other larvae and adults.A plastic prototype trap
was initially designed but was found to be too expensive and
complicated for use in the field. A simpler and cheaper design has
since been operated successfully in field trials in Kenya. This is a
simple cardboard delta trap (D-shaped) coated in a protective layer of
plastic with a pheromone impregnated plastic vial inside the trap. The
trap protects the fungal inoculum inside from damaging levels of UV
light and also helps to maintain the high humidity necessary to ensure
infection of the moths. The traps remain effective for up to one month
when placed on the ground amongst the crop, the inoculum being
refreshed each week.
An alternative fungal pathogen, Beauveria bassiana
(Deuteromycetes), is also being tested for use in the pheromone trap.
Beauveria bassiana spores from the trap are passively vectored
from contaminated moths to other adults and larvae in the population
much more readily that Z. radicans. However, unlike moths
infected with Z. radicans, when moths die on the crop due to
B. bassiana the cadaver does not remain attached to the plant
and usually falls to the ground. Effective transmission from B.
bassiana infected cadavers and the establishment of an epidemic is
therefore less likely to occur than from Z. radicans infected
cadavers.
Judith Pell, senior research scientist at IACR-Rothamsted working on
these fungal pathogens is hopeful that a cocktail of both these
pathogens may be more effective in controlling P. xylostella
populations in the field than either species used alone.
Trap trials have only been conducted in Kenya and Malaysia so far
but it is hoped that the technology will be of use wherever
diamondback moth is a problem. The technology could become an
important tool for subsistence farmers in developing countries
particularly if used as part of an IPM strategy.
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Peppers - in hot pursuit of success
Sweet peppers and chilli peppers (Capsicum spp.) are
renowned world-wide for the diverse taste and colour that they add to
sauces and dishes. Chilli peppers are commonly purchased either as
green or red fruit. They are thinner and longer in shape than their
less spicy relatives but a wide diversity of varieties can be found
(Mexico alone has more than 200 different fruit types, including
malapeno, serrano, New Mexican, ancho, cayenne, and habanero. Pungency
is ranked in Skoville units, and ranges from 0 in a sweet bell pepper
to the notorious "habanero", the hottest pepper in the world
at a blistering 200,000 Scoville units. Sweet peppers are just as
diverse as hot peppers: they are commonly bell-shaped but can vary in
colour, including white, yellow, orange, purple, red and brown, as
well as the common green colour. 
Dr Terry Berke, pepper breeder at The Asian Vegetable Research and
Development Center (AVRDC), Taiwan, is cross-breeding peppers to
enhance yield and quality in existing production areas, and to promote
new varieties which will grow in the hot, humid tropics. For instance,
paprika peppers are traditionally grown in Spain and Hungary, but
AVRDC hopes to create a tropical paprika pepper which could be grown
by Asian and African farmers for export. The programme also hopes to
achieve an extremely hot chilli pepper for extraction of capsaicin
(the chemical which gives chilli peppers their 'burn'),
which can be used in a variety of medicinal products including perhaps
surprisingly, in creams for the treatment of burns.
As with other vegetable crops, the hot humid conditions of the
tropics make the presence of disease inevitable. Peppers are affected
by fungal, bacterial and viral pathogens (as well a variety of insects
pests) so a major focus of AVRDC's pepper breeding programme involves
selection for resistance to pepper diseases which seriously limit
production. In 1997, success was achieved with the first
multiple-disease resistant lines. Four varieties are currently
undergoing final testing and multiplication for release later in 1998.
All four varieties are resistant to CVMV (chilli veinal mottle virus)
and some are also resistant to PVY (potato virus Y), two serious
diseases of peppers in Asia. Resistance to bacterial wilt (Ralstonia
solanacearum) and Phytophthora capsici have also been bred
into some of the four lines. "By providing multiple disease
resistance in our new pepper lines, we are hopeful that the four
varieties will provide farmers with a greater choice depending on the
disease problems of their region," says Dr Berke.
The multiple-resistant pepper lines have been developed at AVRDC's
main headquarters in Taiwan for use in Asia but work on peppers is
being expanded to AVRDC's regional centres in Tanzania and Costa
Rica. Surveys are currently being undertaken in each region to assess
the fruit types preferred and to determine the main constraints
limiting production. Once this initial work is complete, the
information will be used to determine the direction of future
cross-breeding efforts for peppers in Africa and Latin America.
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Micronutrients: releasing the secret
More than 2 billion people worldwide suffer from micronutrient
deficiency: a lack of those essential minerals and vitamins in the
diet which keep the body healthy and help prevent disease. Lack of
such essential dietary components has led to impaired physical and
mental development in young children and an increased mortality,
particularly in women in developing countries. Vitamins A and C, and
iron and iodine are particularly important as micronutrients which
help to provide a balanced diet for an active life.
Vegetables are a major and efficient source of micro-nutrients when
compared to cereals and meat products. However, most vitamins and
minerals are sensitive to the way they are handled, stored and
processed during cooking. The greatest amount of nutrition can be
obtained if vegetables are eaten fresh since cooking and storing may
lead to nutrient degradation. But, the bio-availability of some
nutrients, such as iron levels in mungbean, may still be present
during and after cooking but not released in a form suitable for human
digestion.
To bring out the goodness in mungbeans, AVRDC has teamed up with the
Home Science University for Women, Coimbatore in Southern India to
discover the best ways in which to prepare and cook this well-used
vegetable. Tests have already shown that the addition of ascorbic acid
to mungbean recipes will increase the bio-availability of iron and
that allowing the sprouting of mungbean seeds will do the same.
Work is currently being funded by USAID for a Home Science
University researcher, Dr Amirthaveni Subrama Nian, to work at AVRDC
to test certain recipes using common sources of ascorbic acid, such as
fruits, tomatoes and leafy vegetables, to cook with mungbeans to
release the essential iron. For instance, leaves of drumstick, a
common ingredient to Indian recipes, has been tested with mungbeans to
make a dahl. The addition of green tea leaf, coconut and cumin
complete the flavoursome dish. This, and twenty other recipes are
being cooked and tested for their bio-availability of iron over a
three-month period. The dishes with the greatest amount of iron will
then be selected and tested in villages in India for their
acceptability and their ability to improve the iron levels of test
groups of 100 school children and 100 young women over a period of
time.
Once the trials have been completed and certain recipes deemed to be
a success, information about the recipes and their nutritional value
will be disseminated in India and will be available for use elsewhere.
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Vegetables crop losses due to insects can be as high as 25% and the
loss of potential income is severe for small-scale farmers. As a
result, pesticide usage has increased dramatically over recent years
but extensive damage has still occurred as insect resistance to the
chemicals has increased. Pesticide use is often high just before
harvest, resulting in consumer concern that residues may still be
present in produce on sale. Farmers exporting vegetables, such as
French beans, peas, okra, capsicums and aubergines are finding
consumer demands for blemish-free products but stricter regulations on
pesticide usage an increasingly difficult challenge.
Traditionally, farmers have sprayed their crops at regular intervals
throughout the cropping season whether the insects pests were present
or not. Not only is this expensive, wasteful and environmentally
damaging, it has also resulted in the insect resistance which has
become so widespread. Pest control can be achieved just as effectively
if farmers learn to routinely inspect their crops (scouting) and to
limit their pesticide applications. In addition, a sufficient period
(the harvest interval) should be left between the last pesticide
application and time of harvest to avoid the occurrence of chemical
residues in the produce.
Based in Nairobi, Kenya, The International Centre of Insect
Physiology and Ecology (ICIPE) is helping farmers to adopt Integrated
Pest Management (IPM) measures as a more effective method of pest
control. Biological control is an important aspect of IPM but farmers
who still need to use pesticides are encouraged to use them in a more
limited and rational manner. Dr Sithu Sithanantham, an ICIPE
entomologist, is also keen to promote nature's role in reducing
pest populations: natural enemies or predators which feed on the
insects. "In the past natural enemies, which are themselves
insects, were killed off by the liberal spraying of non-selective
chemical insecticides," he says and advises, "Farmers should
be encouraged to use milder, more selective chemicals designed to
specifically target the crop pest, to use them in smaller amounts and
finally, to positively conserve the habitat of the beneficial insects."
To further avoid problems of insect resistance and chemical
residues, farmers could opt to use a botanical insecticide, such as
Neem, (Azadirachta indica) or to avoid the use of chemicals
altogether by adopting a variety of cultural practices, such as the
planting of 'trap' crops. For instance, problems of whitefly
(Bemesia tabaci) damage in capsicums can be reduced by
planting bean seedlings, which are more attractive to the pest at
field boundaries. Beans grown as a trap or intercrop must be planted a
few weeks in advance so that they are more attractive to the pest than
the main crop.
The Vegetable Crops IPM Project at ICIPE endeavours to incorporate
and integrate the most compatible alternatives for farmers to use in
their production of high quality vegetables. The project has been
initiated in Kenya, as a major exporter of vegetables, but aims to
broaden its coverage to other countries in Eastern and Southern region
of sub-Saharan Africa.
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Good companions
Companion cropping is used by the Bangladesh NGO Proshika to deter
pest attack in fruit and vegetable crops grown at their Ecological
Farming demonstration and training site near Dhaka. One row of onion
sets (bulbs) planted between every two rows of cabbage can repel
cabbage lepidopterous pests (butterflies and moths). A similar result
can be achieved by planting garlic bulbs or sowing coriander seed
between brassicas. In all the cases the smell of the intercrop will
tend to confuse the brassica pest. Similarly, where the pomegranate
moth is a problem, attacking the fruit and making it unsaleable,
transplanting sugar apple close to pomegranate will again confuse the
pest and in many cases save the fruit from attack.
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LIFTing horticultural production
Liquid fertilizer, prepared by fermenting leaves of leguminous trees
with water in a sealed container, is part of the bio-intensive
horticulture practised by the LIFT Project in Bangladesh. LIFT (Local
Initiatives for Farmers Training) encourages women to optimise
cropping of their small landholdings (homestead gardens) in order to
improve family nutrition through increased vegetable consumption, and
income from sale of surplus produce.
In order to maintain soil nutrients under intensive cropping, leaves
of leguminous shrubs and trees such as ipil-ipil, glyricidia, leucaena
and sesbania are placed in drums with water and are sealed for
fermentation to occur in anaerobic conditions. After 15 days the
leaves have rotted and the concentrate is ready for further dilution
with water prior to application to plants.
LIFT avoids use of chemical fertilizers and pesticides on the
principle that natural soil conditions will provide optimum growing
conditions and that healthy crops will be predisposed to resist or
shrug off pest attack. If pest attack does occur, botanical pesticides
such as neem (Azadirachta indica) seed and leaf extracts are
preferred treatments. Cow urine is also claimed to be successful. In
other instances, hand removal of pests or infected plant parts may be
appropriate. Pest management is the objective rather than pest control
and LIFT extension staff promote the concept that not all insects are
harmful and that healthy crops often recover from pest attack. This is
the same approach as that promoted by Farmers Field Schools in Asia
and Africa.
LIFT homestead gardeners growing vegetables, fruit and seedlings of
fruit and timber trees for sale have generated an average annual
income of some Taka 8000 (US $ 200) whereas prior to the project they
had little or no income. In addition, family consumption of fruit and
vegetables has also increased benefiting the health of children and
adults.
The LIFT Project is supported by CARE International (Bangladesh).
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Onion and garlic
Onion (Allium cepa) is second only to the tomato in terms of
its economic importance as a vegetable. Onion and its relatives,
garlic, shallot, the Welsh onion and chives, are used in salads,
sauces, flavouring dishes, and pickles. Most varieties of fresh onions
will store well for several weeks and, if dried, they will last for
longer. The Asian Vegetable Research and Development Center (AVRDC)
has now developed lines of onion that will store fresh for up to five
months, and improvements have also been made to some lines which are
now heat tolerant.
A major focus of AVRDC's work on onions is breeding for
resistance to the major diseases which affect productivity. Developing
resistance to Stemphylium leaf blight and anthracnose (Colletotrichum
gloeosporioides), both of which are important economic fungal
diseases of onions, has been difficult as no reliable source of
resistance is available in onion. However, its close relative, Welsh
Onion (A. fistulosum), was found to have resistant genes to
both diseases and work is now proceeding to cross these genes into
A.cepa. Some promising combinations have been obtained.
The Welsh onion has also been found to have good heat tolerance and
resistance to a variety of other tropical and temperate onion
diseases. These valuable genes could provide further possibilities for
future improvement to onion lines at AVRDC.
Another relative of onion, garlic (Allium sativum), is a
popular ingredient in dishes around the world and it has long been
revered in history for its medicinal properties. Yet productivity in
the tropics is low despite the wide variation available in modern
garlic cultivars. The problem lies in the propagation of garlic: A.sativum
is sterile so seeds are not produced after flowering and regeneration
relies on vegetative propagation (cloning). AVRDC's improvement
programme for garlic is focusing on clonal selection and mutation to
provide lines with good bulb yield and quality traits, such as
resistance to viral diseases which cause extensive losses in garlic
yields. Encouraging results were achieved last year with several lines
demonstrating good bulb yield. The selected clones will be further
evaluated and multiplied for large-scale testing at a number of sites
in different regions.
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"Saffron" tomato for health?
Most tomato varieties (Lycopersicon esculentum) are deep red
in colour and have round fruit which can be cooked or eaten fresh and
are popular worldwide. Others have elongated fruit (plum tomatoes)
which are used for canning and are more common in the Mediterranean
region. Whatever the shape, tomatoes are an important source of
vitamins, although some of these may be lost in cooking. And even raw,
the overall level of betacarotene (pro-Vitamin A) in most varieties of
tomatoes is low compared to other vegetables.
As part of a wide-ranging tomato breeding programme, AVRDC is
working towards a change in colour to elevate the pro-Vitamin A
content of tomatoes to equal that of carrots. Strains of yellow or
orange coloured tomatoes have been bred with six to twenty times more
betacarotene than normal red fruit varieties. However, despite their
greater nutritional value, yellow/orange tomatoes are not proving
popular with consumers in Asia where the fruit traditionally adds
colour to their dishes. Therefore, other types are also being
developed with double the usual betacarotene levels but with a fruit
that retains their characteristic red colour. Preliminary field trials
for a range of coloured tomato varieties are already underway in
Taiwan and it is hoped that a selection of the most successful types
will be sent to Bangladesh for further trials early this year.
Small round varieties (cherry tomatoes) which are particularly
attractive to children for snacking, are a potential source of income
for small farmers, especially on the urban fringe. AVRDC breeders have
already achieved success with a cherry type which was released last
year and is now highly valued by Taiwan's growers. Because of its
potential, breeders are working on a high betacarotene cherry variety.
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Nematodes - a knotty problem
The dilemma posed by the serious problem of root-knot nematodes (Meloidogyne
spp.) is that they are a parasitic pest that is undetectable to
the naked eye. However, their impact is obvious in the severe symptoms
they cause to a wide range of fruit and vegetable crops in tropical,
sub-tropical and temperate climates. The nematode pest, once
established, can cause severe damage to plant roots and seriously
affect crop yield. However, biological control agents (BCAs) currently
under trial, may provide market garden farmers a way of getting
literally to the root of the problem.
Less than a millimetre long, nematodes enter the roots of the
seedling when the plant is most vulnerable. These microscopic animals
cause serious swellings (galls) to form on the roots which prevent
them from taking up water and nutrients. However, the symptoms above
the soil surface are often indistinguishable from those caused by some
fungal diseases, damage done by insects, nutrient deficiencies or even
water logging. It is only by digging up a plant and examining the
distorted root systems, and observing the heavy galling, that the
diagnosis for root-knot nematodes may be made correctly and the
appropriate treatment applied.
Even when correct diagnosis has been made, effective treatment can
still be difficult to achieve. Most vegetable crops and some major
fruit crops are affected by root-knot nematodes but the pest can
persist in the soil for many years, even in the absence of these crops
being grown: many agricultural weeds are also host to nematodes,
acting as reservoirs for the parasite to re-infect susceptible crops
when they are planted. To avoid re-infection a field must be left
fallow for many years but, with increasing demand for land in most
countries, this is not a realistic solution. Rotation is another
possibility: to grow a crop that is not susceptible to root-knot
nematodes.
Unfortunately, very few of these non-susceptible vegetable crops are
of high enough value to be attractive to small-scale farmers or market
gardeners to grow. So many turn to the use of chemicals (nematicides)
to combat the problem. But, rather worryingly, nematicides are amongst
the most contaminating and highly toxic of any agrochemicals that are
used world-wide and, if adequate precautions are not taken, people can
be easily poisoned directly or by the pollution of water courses. And
they are expensive.
An environmentally safer and lower cost solution may lie in an
organism even smaller than the root-knot nematode. Simon Gowen, Senior
Research Officer in the Department of Agriculture, University of
Reading and his colleagues have come up with the idea of using a soil
bacterial inoculant as a bacterial control agent. The inoculant
consists of a powdered form of the bacterial spores of Pasteuria
penetrans - a naturally occurring bacterium which is found in
soils throughout the tropics. The natural levels of this bacterium in
the soil is not usually sufficient to combat the problem of nematodes
but a culture of Pasteuria penetrans can be used to infect the
nematodes and kill them.
The technique has proved successful under trial conditions in
Ecuador, where it is was shown that after only six crop cycles (30
months) plants treated with Pasteuria in the field were
virtually free of root-knot galls. It is now hoped to introduce the
technique to African research stations and extension officers. By mass
producing the inoculum at the research stations, it could then be
given to market gardeners for use with crops such as tomato, okra and
eggplant. The technique may be tedious: it can take up to two months
to obtain sufficient inoculum to incorporate into nursery seed beds,
but it is inexpensive, and more importantly, extremely effective. The
spores of Pasteuria penetrans are resistant to desiccation and
heat so it is likely that once the spores are in the soil, they will
remain for quite some time.
An alternative BCA lies in the nematophagous fungus, Verticillium
chlamydosporium, which has been demonstrated to have considerable
potential for reducing root-knot nematode populations by Brian Kerry
and his team at the Entomology & Nematology Dept, IACR-Rothamsted.
A naturally occurring saprophytic fungus of nematode eggs, V.
chlamydosporium has been shown to effectively reduce the number of
healthy nematode eggs being produced in UK soils by up to 85%.
Testing has also begun on Zimbabwe research stations where the
fungus is being used in a rotation system of tomatoes (1 yr), a major
host for root-knot nematodes, followed by maize, beans or kale (3
yrs), which are grown as non-susceptible crops for this pest. The
advantage of the fungus (applied to non-susceptible crops) is that it
helps to further reduce the nematode populations. Further trails are
also intended for Costa Rica, Ghana and Pakistan where local varieties
of V. chlamydosporium will be used to control nematode
problems in vegetable crops grown in these regions.
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Chinese cabbage - shaping up for heat tolerance
Chinese cabbage, like most brassica varieties grows well in moist,
cool conditions and is traditionally found in the tropical highlands.
But as the popularity of Chinese cabbage has increased, farmers in the
lowland tropics have become more interested in cultivating the crop
for their own culinary use. Heat tolerance has already been developed
by AVRDC in round-headed Chinese cabbages. But, as consumers prefer a
more elongated Chinese cabbage, heat tolerant characteristics are
being combined with the preferred pointed shape. A suitable hybrid
that can be used commercially should be available soon. Eventually it
is hoped that a Chinese cabbage will be developed that is heat
tolerant, high yielding and early maturing. Rich in minerals and
vitamins and high in fibre, Chinese cabbage can be promoted as part of
a healthy diet.
As with any crop, microbial diseases are more prevalent in warmer
conditions and proper management of the vegetable in lowland areas is
therefore essential if good yields of Chinese cabbage are to be
obtained. Dr Dae-Geun Oh, AVRDC breeder in the Chinese Cabbage
programme, is keen to see that good IPM strategies are promoted. "This
should include regular surveying of the crop to determine which pests
or diseases are about to hit the crop. Once a pest or disease has been
identified, farmers should spray on a limited basis with correct
amounts of appropriate pesticides," he advises. (see
Less pesticides, less residues).
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