Frequently Asked Questions

Even among rhizobia that can nodulate the same plant, there are many different, geneti-cally distinct, strains. Some fix nitrogen better—more efficiently—than others, resulting in superior plant growth. Some also compete better with rhizobia that are already in the soil. This means that they can enter the plant’s root hairs more efficiently, resulting in faster nodule formation. Commercial Rhizobia strains have been developed for specific legumes (peanuts, cowpeas, clover etc.)

The fixed N2 is released when the plants die, making it available to other plants and this helps in fertilizing the soil. If the legume crop debris is removed from the field there is less benefit because most of the nitrogen is in the leafy and fruiting parts of the legume plant.

Legumes can fix more than 250 kg N per hectare. However, the amounts of N2 fixed can vary considerably depending on pesticide applications to the soil,
presence and effectiveness of rhizobia, pest damage, plant genotype and age,
plant and rhizobia interactions and changes in soil physiochemical conditions. Nitrogen is an important building block of proteins and the seeds of legumes (beans and peas) are valued for their high protein contents.

A symbiotic relationship between the plant and the microbe requires both the plant and the microbe to benefit. This requires some compromises to take place. For example: For the plant to be able to benefit from the added available nutrients provided by the rhizobacteria, it needs to provide a place and the proper conditions for the rhizobacteria to live. Creating and maintaining root nodules for rhizobacteria can cost between 12–25% of the plants total photosynthetic output.

Plants are able to shape their rhizosphere microbiome by secreting different exudates attractive to different soil microbes. Different plant species host specific microbial communities when grown on the same soil. Rhizobium colonise legume roots but not other types of plants.

Some PGPRs produce phytohormones (e.g. auxin), which promote the formation of later-al roots. Increased lateral root formation leads to an enhanced ability to take up nutrients for the growth of the plant.

PGPR can (a) increase nitrogen availability to the plant (b) precipitate insoluble com-pounds from the soil and sequester these in their own cell components – thereby cleaning up heavy metal pollution in the soil (c) migrate form the rhizoplane to the rhizosphere where they can bind ions in biologically unavailable forms (d) assist in the formation of iron-chelating siderophores to improve the fitness of plants by increasing iron uptake.
(e) reduce the intake of sodium into the plant and avoiding sodium toxicity in high saline soils (f) increase the availability of nutrients to the plant by production of organic acids that change the pH of the soil near the root.

Rhizobacteria are root-colonizing bacteria that form symbiotic relationships with many plants. The name comes from the Greek rhiza, meaning root. Though parasitic varieties of rhizobacteria exist, the term usually refers to bacteria that form a relationship beneficial for both parties (mutualism). The relationship of PGPRs with host plants are either rhizospheric (limited to the outside of roots) or endophytic (within the host tissues). Biostimulants = PGPR = Biofertilisers.

Roots release nutrients made up of organic acids and inorganic hydrocarbons that microbes use as a food source. Soil microbes will colonise hot spots on the roots surface where they can feed on sloughed-off root cap and border cells, mucilage, and plant exudates.

Whilst there is some secondary pick of EPF spores by host insects passing by spores previously deposited on the leaf by commercial spray applications – this is of minor signifi-cance compared to direct application of the spores onto the host insect’s body. This requires optimum application coverage and application at a time of day when the pest is exposed to spray coverage (on top of the leaf).

The parasitic wasps have defense mechanisms, which protect them from infection by EPFs. The EPFs tend to be very specific in their target pest and parasitoids come from very different insect families to most pests. Parasitoids often have the ability to distinguish between hosts that are infected or not infected by EPFs and avoid coming into contact with the EPF.

If the EPF is being used in an auto-dissemination device and is relying on the host to re-distribute the EPF within the pest population – the ADD is more efficient, the longer it takes for the contaminated pest to die. The infected pest will come into contact with more pests, the longer it lives and more pests will receive EPF spores. (see FAQ on ADD).

The speed of death depends on the application rate of the biopesticide, the temperature and the susceptibility of the pest to that particular isolate of the biopesticide. It may take 3 to 10 days for the pest to die, but during this period it may feed less on the crop plant and lay fewer eggs.

The spores of an entomopathogenic fungus must land on a target pest, which it recog-nizes as a host before it will even germinate. Once it has germinated, it will grow over the surface of the host in search of a soft area where it is easier to penetrate the insect pest. It then forms a special structure, called an appressorium, which helps it to drill down and enter the pest’s body. Once inside the pest, it must be able to overcome the pest’s own immune system in order to enter the insect’s haemolymph. The insect’s haemolymph is full of nutrients, which feed the EPF allowing it to grow. In the process of growing, the EPF gradually kills the pest and eventually the fungus grows and sporulates on the outside of the dead insect.

Although biopesticides are killed by UV light (after two days), for practical reasons they need to be applied at any time of day when a chemical would have been applied. It is more important to apply the EPF when the pests are active in the crop and can be targeted by the biopesticide spray. Nocturnal pests (weevils and moths) should be sprayed at night. Fruit flies should be sprayed between 9 and 1 am and 4 and 5 pm. Thrips should be sprayed within two hours of sunrise and within 2 hours of sunset when adults congregate on the outside of flowers and the upper surface of leaves.

If the temperatures are low and the crop is dormant, the life cycles of the pests and diseases are probably also ‘dormant’. In which case, it is probably not cost effective to continue applying the biopesticide/biofertiliser. However, the farmer should monitor soil temperatures and soil moisture levels and should consider stopping biofertiliser/ biopesti-cide applications to the soil if the winter soils are too wet to sensibly apply irrigation water or the soil temperatures are below 5 deg C. Once the spring soils begin to dry out and the soil temperatures rise above 5 deg C, the farmer may consider re-starting the regular soil application programme.

The lower the level of organic matter in the soil – the more frequently the biopesticide should be applied. Both EPFs and Trichoderma can grow saprophytically on organic matter in the soil.

Yes. The biopesticides will colonise the roots of the crop, even in pumice grown crops. There will also be some dead roots (organic matter), which will be a substrate for saprophytic growth of the beneficial microbes. Weekly applications are recommended. If this is too expensive – apply half rate doses.