Fact sheet on controlling deadly potato cyst nematodes

By George Munene

Using ar­ti­fi­cial and nat­ural nematicides small­holder farm­ers in Kenya can con­trol the potato cyst nem­at­odes (PCN), a pest that causes yield losses by at least 80 per cent.

In a study of 20 potato grow­ing counties in Kenya Potato Cyst Nem­at­odes (PCNs) were ob­tained in 71.8 per cent of the counties with Nyandarua County at 47.6 per cent re­cord­ing the highest PCN field-incidence.​The situ­ation is fur­ther com­poun­ded by the fact the Shangi potato vari­ety grown by 65 per cent of farm­ers for its shorter dormancy and cook­ing time is the most sus­cept­ible to PCN.

Pota­toes are the second most con­sumed food crop after maize in Kenya, however, pro­duc­tion has been on the de­cline: on a land­mass size of 133,532 hec­tares, the coun­try pro­duced 1.9 mil­lion tonnes of pota­toes in 2015. Over a cor­res­pond­ing period in 2017 however, Kenya’s potato pro­duc­tion fell to 1.5 mil­lion tonnes off 192,341 hec­tares.

With Nyandarua being the coun­try’s potato bas­ket, ac­count­ing for up to 40 per cent of the  total pro­duc­tion of pota­toes, farm­ers in the county now pro­duce 18 bags, from ¼ acre, down from the pre­vi­ous 30 bags.

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PCN, first re­por­ted in Kenya in 2015, is clas­si­fied by KEPHIS as a quar­ant­ine pest (a pest sub­ject to of­fi­cial con­trol and reg­u­la­tion due to the po­ten­tial eco­nomic dev­ast­a­tion it may present to an area) can lie dormant in soils for up to 20 years and in­fects 82 per cent of pota­toes grown across 22 counties.

Potato plants af­fected by the PCN ex­hibit the fol­low­ing char­ac­ter­ist­ics:

1. leaf dis­col­or­a­tion/yel­low­ing and wilt­ing
2. root with cysts
3. un­even tuber sizes on one potato plant
4. re­duc­tion in the num­ber of roots
5. dwarf­ing of potato tubers and the plant and, 
6. re­duc­tion in the num­ber of crops

The nem­at­odes caus­ing PCN are mi­cro­scopic worms, meas­ur­ing less than one mil­li­meter in size, mean­ing they are only vis­ible through a mi­cro­scope lens. As these symp­toms closely mimic water and nu­tri­ent de­fi­ciency in pota­toes, farm­ers are often left none the wiser on what is caus­ing a re­duc­tion in yields. 

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Potato cyst nem­at­odes are spread by:

1. Im­port­a­tion of in­fec­ted plants or plant ma­ter­i­als and
2. loc­ally, by the spread of cysts through soil, wind, water and vehicles

The total pro­duc­tion of basic potato seeds in Kenya stands at 6,700 met­ric tonnes, short of the 30,000 met­ric tonnes re­quired. Ac­cord­ing to KEPHIS, the in­formal sec­tor in Kenya ac­counts for about 70 per cent of potato seeds propag­ated by farm­ers across the coun­try, this makes the curb­ing of PCN al­most im­possible.

The easi­est way of con­trolling potato cyst nem­at­odes is the use of nematicides, which are chem­ical pesti­cides used to kill plant-para­sitic nem­at­odes. Most nematicides are however burned for being toxic to the en­vir­on­ment. 

Nematicides sold in Kenya in­clude:

1. NEMATHORIN® 150EC
2. Ad­ven­ture® 0.5% GR
3. Alonze® 50EC
4. Farmchance® 250 EC

Stud­ies done in Kenya by sci­ent­ists from The In­ter­na­tional In­sti­tute of Trop­ical Ag­ri­cul­ture have found that cov­er­ing potato plant­ing seeds with ba­nana paper laced with min­imal doses of pesti­cides dur­ing plant­ing de­ters PCN’s from reach­ing the potato seeds.

In stud­ies by the Uni­versity of Flor­ida Food and Ag­ri­cul­tural Sci­ences Ex­ten­sion de­part­ment crop­ping in Marigold flowers into pota­toes/cab­bages has also been sci­en­tific­ally proven to re­duce nem­at­ode at­tacks as they act as an al­tern­at­ive host for the nem­at­odes, but pro­duce nat­ural com­pounds that act as nematicides. This kills the nem­at­odes pre­vent­ing them from breed­ing. In time, the nem­at­ode pop­u­la­tion slowly de­creases.