Kūmara pests and diseases
Agronomist Dr Nick Roskruge has expert advice to protect your crop.
K umara evolved from a climbing vine and is widely believed to have been domesticated around 6000BC.
Once the kumara¯ crop is established, it is highly tolerant to long periods of drought, although the yield and quality might be affected.
Even though frequently associated with the potato (Solanaceae family), the kumara¯ (Convolvulaceae family) is not botanically related at all. The main similarity between these root crops is the same-sized edible “tuber” which they produce. The kumara¯ is a specialised swollen root rather than a true tuber, though conventionally it is referred to as one.
Botanically, sweet potato is closely related to the convolvulus which is often seen growing rampant around New Zealand.
An ancient crop
Sweet potato or kumara¯ ( Ipomoea batatas) is one of the few species of the genus Ipomoea which develops fleshy and palatable storage roots. Kumara¯ evolved from a climbing vine and is widely believed to have been domesticated around 6000BC, with its botanic centre of origin in the Andean region of South America; the highlands of the continent including Ecuador, Peru and Bolivia.
It is a more recent arrival in the South Pacific. The route taken from the South American continent on the eastern border of the Pacific Ocean to the Polynesian, Micronesian and Melanesian islands is not definitive in either tradition or science, but the relationship of the plant to Polynesia is beyond question and spans centuries and many migrations.
Kumara¯ can be grown year-round in the tropics and is propagated vegetatively by planting stem cuttings from which new plants develop. However, in Aotearoa, it is a summer crop requiring winter storage and the production of cuttings (tipu or tupu) in spring to regenerate the crop.
The crop requires heat and welldrained soils, and high nitrogen levels in the soil are detrimental to the development of the tubers. Once the kumara¯ plant is established, it is highly tolerant to long periods of drought, although the yield and quality might be affected.
The main concern in kumara¯ production is their handling and storage, as they must be cured after harvest and be kept under quite strict conditions in order to store well. Without grading and curing, the tubers’ disease vulnerability and susceptibility to damage during storage can cause high losses of the total crop.
Modern cultivation
Crop health is important in any food system. The healthiest of crops withstand pest and disease as well as environmental pressure much better than unhealthy plants.
For kumara,¯ the quality of each year’s crop depends entirely on the selection of kumara¯ used to produce the tipu. At the end of each harvest, the best quality kumara¯ based on key characteristics should be sorted and retained for the future tapapa¯ or p¯arekereke (tipu bed). This is based on the principle that the qualities of the parent generation which contribute the cuttings to be planted have the preferred features for the future crop. Characteristics to guide selections are: no apparent disease or pest damage; uniform size; shape; colour; and sourced from high yielding plants in the field.
These kumara¯ need to then be carefully cured and stored, then regraded prior to setting out the tipu beds in spring.
For kumara,¯ there are many plant pest and disease issues that can affect them and growers need to be aware of their impact on crops during the production or storage periods.
Before we delve into the plant health issues, we should recognise that some insects and biology are actually very beneficial for these crops. Beneficial organisms (nga¯ koiora pai) assist nature to find its balance through establishing biological populations which interact with each other. Beneficial insects are generally those such as ladybirds, lacewings, tiger beetles and some parasitic wasps that prey on pests.
We also think of pests as mostly small creatures but the large pests such as rabbits, hares, rats and some birds such as pheasants or pukeko¯ cause considerable feeding damage to kumara¯ crops.
Pest problems Pest insects (ng¯a p¯orearea)
are generally classified because of the damage they create, their role as vectors or carriers of disease pathogens or the weakening of the plants their feeding does. For kumara,¯ the pests are mostly a problem during field production whilst diseases are most often a problem in storage.
One example of a long-standing pest on kumara¯ crops is the convolvulus hawk-moth ( Agrius
convolvuli), in particular the larva or caterpillar stage known as the mokowhiti or awhato.¯
This caterpillar is the most acknowledged pest of the pre-contact Maori¯ gardeners. It is said to have arrived periodically in hoards without warning and was handpicked from the crop to be burnt on the side of the field. The caterpillar has several names and a distinctive horn on its head. It relishes the leaves of kumara¯ and is known for its constant nibbling, starting at the leaf edges and leaving just the leaf veins and midrib intact. Traditionally, the caterpillars were picked from the crop and destroyed off-site or birds were encouraged into the fields to feed on them.
The pupa or cocoon is easily identified with a long and curled proboscis or protruding tongue. The adult moth was noted as often seen feeding from the nectar of the hue or gourd flowers.
Today, the use of pesticides is possible but they need to be used cautiously to target the appropriate lifestage of the moth and to ensure predatory insects or spiders are not inadvertently targeted.
Storage issues
A range of storage rots can affect kumara¯ – most are fungal in origin but some are bacterial.
Fungal diseases (nga¯ kopurawhet¯u)¯ are a major problem, both in the field and in storage. Fungal disease organisms obtain food by absorption from their hosts, and can be host and environmentally specific, requiring conditions such as temperature and humidity that encourages their growth and spread. Given the right conditions, fungal diseases are often fatal to their host plants.
Bacteria (nga¯ kitakita) are a group of minute organisms, much smaller than fungal spores, that are often pathogenic, causing rots, cankers, blights and galls in plants. There are many types of bacteria, some are functional (for fermenting in the food industry, for example), and others are destructive but bacterial diseases are generally fatal in crops if they are allowed to spread.
Two examples of kumara¯ disease which may be familiar are soft rot ( Sclerotinia sclerotinium) and scurf or parapara ( Monilochaetes infuscans).
(See the illustrations on the left.)
These examples remind us of the vulnerability of our crops, and I suspect the effect of climate change is also being felt by many growers.
The seasons are becoming more fluid and this can contribute to increased plant health issues, especially over extended seasons. A good kumara¯ grower should be aware of possible issues and see them early so they can respond early.