Diseases/Plaagbeheer
Insette/Produksie
43
February 2014
Even when these spots become larger than their original sizes, they still
remain smaller compared to the other two diseases in general.
A narrow yellow or light green zone surrounds the lesions and they
later dry up, with the dry area falling down, thereby leaving holes on the
leaves. The watery spots also appear on the pods with a circular shape,
later becoming darker green and sunken brown, and causing the infected
area on young pods to bend at point of infection.
When the seeds are infected, they first develop a watery appearance at
the point of infection and later become brown and shrunken when the
disease becomes severe. When bacterial brown spot develops all over
the plant, damage also appears on the stems. Sources of these bacteria
include infected seeds and volunteer beans, weed hosts and plant debris.
Management of bacterial diseases
The very first thing should be every dry bean producer’s aim, and that is
to plant disease-free certified seed as a primary step in managing these
seed-borne diseases. However, bacteria may still find their way to get to
your crop through other means.
For this reason, planting dry bean cultivars that are resistant (if available)
to these bacteria is recommended as this serves as an effective method
of controlling bacteria. Since previously bacteria-infested beanstalks
may still be present in the soil, dry beans should not be planted on the
same piece of land for at least two years.
Other crops, such as oats, barley and wheat may be planted during this
two-year period due to the fact that these crops are non-hosts to these
bacteria, thereby eliminating bacteria. Producers must remember that
bacteria are capable of surviving on dry bean leaves without causing
diseases on the crop until such time as conditions are favourable for
infection.
Bactericides or chemicals that contain copper can accordingly be
applied preventatively during the late vegetative or early flowering
stages, especially in areas prone to bacterial diseases. Once the bacterial
symptoms are already visible, it is already too late for the chemicals to be
effective and it will only control the spreading of the disease.
Debris of infected fields can be burnt or the debris buried as to remove
inoculum sources from problem fields. The weeds and volunteer beans
may also harbour the bacteria, and should be controlled accordingly. Dry
bean bacteria require moisture in order to grow and multiply quickly.
It is for this reason that working in the fields should be avoided during
wet weather and human, animal or machinery movement should be
minimised or completely avoided. These movements can easily spread
the bacteria through the field, especially if plants are wet.
Bacterial diseases of dry beans:
Every producer’s nightmare
Continued from page 41
Gasheerplantweerstand kan gedefinieer word as die gevolg van plant-
kwaliteite wat lei tot ‘n plant wat minder skadesimptome toon as ‘n
plant sonder daardie sekere kwaliteit. In praktiese landbouterme kan
gasheerplantweerstand gesien word as byvoorbeeld ‘n insekweer-
standbiedende kultivar wat ‘n hoër opbrengs lewer as ‘n vatbare kultivar
wat deur insekte beskadig word.
Insekweerstandbiedende plante beïnvloed die verhouding wat daar
tussen ‘n insekplaag en die gasheerplant is. Hierdie verhouding tussen
insek en plant kan geaffekteer word deur drie tipes weerstande:
Antibiose, antixenose of toleransie.
Antibiose-weerstand beïnvloed die biologie van die insek sodat die
plaaggetalle verminder en minder skadesimptome getoon word as by ‘n
plant wat nie weerstand het nie. Antixenose-weerstand affekteer die
gedrag van die insekplaag deurdat die insek verkies om nie op die plant
met weerstand te voed in vergelyking met ‘n plant sonder weerstand
nie. Toleransie is waar die plant skade kan weerstaan of van skade kan
herstel wat deur ‘n insek aangerig is.
Die interaksies tussen insekte en hul natuurlike vyande is noodsaaklike
ekologiese interaksies wat ‘n groot bydrae lewer tot die beheer van
insekpopulasies wat bekend staan as biologiese beheer (
Foto 1
).
In situasies waar hierdie natuurlike interaksies versteur word, kan
potensiële insekplaagpopulasies ontwikkel, wat tot so ‘n mate groei dat dit
lei tot ‘n plaaguitbraak. In meerjarige gewasse, soos aangeplante woude
en vrugteboorde, is biologiese beheer meer suksesvol aangesien die
omgewing nie so drasties verander nie en daar ‘n voortdurende interaksie
tussen die insekplaag en die natuurlike vyande is. In eenjarige gewasse
is die situasie egter anders, aangesien ‘n onstabiele omgewing geskep
word waar die plaag en die natuurlike vyande van mekaar geskei word.
Hierdie skeiding kan soms verminder word deur beter bestuurstegnieke
toe te pas om natuurlike vyande te bewaar en die stabiliteit van die
gewasproduksiestelsel te bevorder.
Parasitoïede parasiteer slegs ‘n ander insekgasheer gedurende sy
onvolwasse stadiums wanneer die larwe ontwikkel, binne (endoparasiet)
of op (ektoparasiet) die gasheer. Gedurende larwale ontwikkeling in die
insekgasheer, word die gasheer gewoonlik heeltemal of gedeeltelik gevreet
en vorm die parasietlarwes papies binne-in of naby die gasheer.
Predatore voed op alle stadiums van hulle gasheer – op eiers, larwes,
papies en volwassenes. Elke predator benodig ‘n aantal individue
van die gasheer om volwassenheid te bereik, anders as in die geval
van parasitoïede wat slegs een gasheer benodig. Die onvolwasse en
volwasse predator moet hul gasheer soek, vind, oorwin en dan eers
begin hulle voed.
Kulturele beheer van insekplae is wanneer die omgewing in so ‘n mate
gemanipuleer word dat heersende omstandighede ongunstig gemaak
word vir die insekplaag. Hierdie manipulering kan deur baie tegnieke
gedoen word, soos deur wisselbou van gewasse, tussenverbouing,
manipulering van plantdatums en bewerking. Ander beheertegnieke
sluit in byvoorbeeld die gebruik van semio-chemikalië (byvoorbeeld
feromone) om insekkommunikasie te versteur.
Dit is egter belangrik om kennis te dra van al die bogenoemde
beheerstelsels wat gebruik kan word om insekplae effektief en ekonomies
te beheer. Dit is dus noodsaaklik omnie slegs op een enkele beheertegniek
te steun nie, maar eerder op ‘n kombinasie van beheermetodes om ‘n
plaag te beheer, dit wil sê geïntegreerde plaagbeheer.
FOKUS EERDER OP
geïntegreerde plaagbeheer
Vervolg van bladsy 40
