Circular [Feb. 1929]

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Georgia State Board of Entomology
M. S. Yeomans, Stat e Entomologist, Atlanta, Ga.

CIRCULAR 41

FEBRUARY, 1929

Preventing Losses From Sweet Potato

Diseases

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By O. C. Boyd, Plant Pathologist
Georg ia State Board of Entomology

Practically a ll varieties of sweet potatoes, inc ludi ng t he Port o Ric o an d ot hers commonly grown in this state, a r e subject to at leas t three ve ry common and often destructive dis eases. Each disease is caused by a separate and di stinct parasitic fungus whi ch is able not only to atta ck the living plant, but also to live in our soils for sever al years as a saprophyt e -that is, able to live on vegetable matter in the soil other than the living cr op plant it se lf. This latter fact accounts in part for a r athe r wid e distr ibuti on of the disease organisms in the state, and makes the control of th e diseases some wha t mo re difficu lt than wo uld be the ca se if the parasites were limited in their feeding habits to th e growing or living potato plant.
Notwithstanding th e wid e distribution of th e dis ease organisms In our soils and their sapr ophyti c habits, effective and pr ofita ble control measures for all of th ese potato troubl es have be en put Into use not only in this state, but In others as we ll. As a matter of fact, ev e ry farmer, whether he grows a small patch for table use only or a la rger acreage for the market, or whether he Is an exten siv e plant grower, can reduce to a minimum the los se s that generalIy occur In this cr op by using a few pr eventive m easures that will be mentioned In connection with the description of the diseases. The following dis eases are the principal ones

1. Stem-rot. This disea se causes a s tunti ng, yellowing and slight wilting of the plan t In both th e be d and the field . The affect ed draw or r unner s hows r ed dish-brown s tr a nds wh en split lengthwise, ca use d by th e fungu s growing In and stopping up th e wat er-conducting vessels. Th e diseased plant in the field usually shows a s hor t, thi ck en ed runner with a cracke d, swollen conditi on at or n ea r the ground lin e. Th e diseased draw may survive the bed, but ca uses a partial or total loss in th e field . Potatoes produced in a stem-rot hill do not commonly rot In th e soil or in st or age, but do cont ain the fungus on th e inside where the disease Is ca r rie d through th e winter storage and Into th e plant bed the folIowlng spring. Stem -rot can be prev ented largely by selecting seed potatoes from fields fr ee from th e disease. If the disease should ap pear in the bed, the infected spo ts shou ld be removed and destr oyed. On ly enough disease-free draws sho uld be set out to furnish vine cu ttings for th e main crop. If only th e outer half of th e runner is taken for th e cut ti n g, there is a large cha nce for escaping any infection that might be pr esent in the draw or the base of th e runn er.
2. Black-rot. Black-rot produces a storage rot of potatoes a s welI as a dis -

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ease of the plan t in the be d a nd in the field . T he para siti c fungus ca uses la rge, more 0 1' less circ ula r bro wn-to-bla ck s pot s on th e potato whi ch re ma in rather firm a nd dr y und er pr oper storage con diti ons .

ding . Li ghtl y affected roots, if bedded. m a y rot com ple te ly in th e bed o r m a y survive to pro duc e a di seased cr op of draws. Such dra ws may die outr ight in the bed or serv e to conve y t he dlsease

T wo COl u m n n s to r ll J,:'c ruts : I ~t rt , Hu rt rut . s h u w i ll l{ muuld)" J;"rowth uf th e eu s u u ! fUIlJ,;"IlS . night , -Iu vu bl a ck-rot , ca ti si llJ,:' II dr~' , wrinkl ed , wart , " pn tut o,

In warm mo ist s to r age, h owe ver, the s pot s may enla rge gre a tl y, and extend deeply int o the potato, th us ca us ing a se r iou s storage rot and r endering the pota to unfit for consu m ptlon or for bed-

to t he field. Diseased pla nt s in the field, a lso , may die, ca using a mi ssi ng hill, or may pa r ti all y reco ver a nd produ ce a hill of potatoes. T he la tter when dug usually show the charac te risti c black-rot s pots ,

an d ma y thu s ca rry th e dis ea s e in to s to rage or th e market.
In the cont r ol of both bla ck-rot and s te m-r ot, t he g ro wer should observe the foll owi ng pr actices : (1) After the cro p is du g in th e fa ll, di s card a ll cut , bruised and s po tte d pota toes, and s tore th e r eo mainder wh ere pot atoes ha ve not pr evi ou sl y been bank ed , bedded or gro wn. (2) Stor e se pa ra te ly th e po tatoes fr om differen t fields a nd from diff erent sources of

crop. (5) In ca se ste rn-rot or bla ck -rot is found in th e fie ld, do not plant pot a to es in tha t field for a period of at lea s t five years.
3. Bed-rot. This di sea se, like black ro t, a ffec ts both th e potato as a storage rot a nd t he plant in th e bed, bu t does not a ffec t the older plant in t he field. It is ca use d by a fun gu s that a tta ck s a number of oth er tru ck a nd field cro ps , as well as m an y orna me nt a ls , throu gh on t th e

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dr a ws or vin e cu ttings, thus a vo iding the mixing of cle a n and di s ea sed lot s. (3) Wh en th e banks are op en ed for bedding, ca refully sort out and dis card a ll roots showing a ny signs of r ot s, and di sinfect th e remainder by s oaking for te n min ut es in a soluti on of four ounces of cor ros ive s ublimate (m ercuric chlor ide) to 30 gallon s of water. (4) During dr aw pulling, destroy all plants that show di seas e, a nd set out on ly eno ugh draws t o furni sh su ffi ci ent vine-cutting to se t t he main

Coasta l Pl a in s a re a. It is parti cu larly adapted to warm, moi st soil s ri ch in organ ic matter, a nd is a ble to live sa pro ph yti cally in s uc h soils for a lon g period of tim e. In m oi st, poorly ventilated hanks , t he di sea se affec t s the s weet potato as a se m i-soft rot, and m a y s prea d r apidl y t h ro ug h th e lot to destro y a la rge per ce nt of the roo ts . In th e bed , the fungus may r ot both t he potato a n d the ba sal part of th e draw, a n d spr ea ds in th e bed to involve large area s. Th e a ffecte d

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draw s wil t, turn ye llowish . a nd sh ow a cha ra cteris ti c yellowish -br own rotting of the low e r en d. The use of or gani c m an ures , a s cot ton-see d m eal , s t a ble m anure,

wh ich ca n not tol emte a n a lka lin e soil. Digging out a nd des t roying diseased s pots in the bed helps to che ck th e s prea d of th e di s ea se th er e.

I4t> ft. black..ro t In f ec ti on o n th e d ra w ; r lJ:'ht. on th e potato.
etc ., sho uld be avoided . Adding lim e to th e bed soil before bed din g at th e ra te of six to e igh t to ns per a cre , ha s hee n fo un d to reduce the los s trom t h is orga n is m ,

4. Sto rage Rots. Su ch s tor a ge rot s a s J av a bla ck-rot, cha rcoa l rot, a n d so ft rot m ay ca use a s mu ch loss to a year's cro p a s anyo ne or a ll of the dis eas es m enti on ed abov e, Storage con diti ons in the ban k or hou se de t er min e largel y t he am ou nt of dama ge fr om t hese fu ngous dis ea s es. Ye t , th e ma nn er in whi ch the pota toes a re han dle d during digging a nd s tor ing is e qua lly a s importa nt. If the pota toes a re cut a nd b ru is ed duri ng digg ing, a nd a re not a Ilo wed to dry prop erl y be for e stor ing, co ns ide rable los s m a y be expe cte d in storage, es peci a Ily if th e bank or hou se is not properly ventilated, and heated in ca se of th e s t orage hou s e. Th e foIlo wing pr e cau ti on s s hould be obs e r ved in o r de r to r e du ce to a m in im um los ses t hat com mo n ty occur in s tora ge: (1) Avo id cutti n g and bru is ing t he root s du ring dig gi ng, a nd a llo w the m to dry tho rou gh ly befo re stor ing. (2) Sort out a ll cut a nd bruis ed ro ot s befo re s to ring. (3) Sel e ct a new s ite for th e banks; a nd disinfe ct th e in t erior of the old s tor age hous e by s pra yi ng with one o f th e foll owing : Cor rosive s ubli ma te, 4 ounces to 25 gaIlo ns; COPIJel' sulphate, 12 ou n ces to 25 gallons; formalin , 1 pint to 25 gaIlons ; or the winter s t rength of lim e-sulphu r fru it tree s pra y. T he in te ri or of the hous e should be r a llow e d to dry before the potato es e n te r . Both banks a nd hous es should be cons t r uc ted with a de qua te ventil a ti on.

R eprin t ed fro m Ag r icult u ra l Bulle tin, A. & W . P . R. R.