Compendium of Potato Diseases - (PDF, 101 mb) - USAID

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2) Powdery mildew is rarely a problem on potatoes grown
under sprinkler irrigation. A heavy rain will also stop progress
of the disease.
Selected References
DUTT, B. L., R. P. RAI, and i1. KISHORE. 1973. Evaluation of
reaction of potato to powdery-mildew. Indian J. Agric. Sci.
43:1063-1066.
ROWE, R. C. 1975. Powdery mildew of potatoes in Ohio. Plant Dis.
Rep. 59:330-33 1.
(Prepared by R. C. Rowe and G. D. Easton)
Early Blight
This disease is found worldwide wherever potatoes are grown.
Symptoms
Initial infection is most frequent on lower, older leaves.
Lesions first appear as small (1-2 mm) spots, dry and papery in
texture, later becoming brown-black and circular-ovoid as they
expand. Advanced lesions often have angular margins because
of limitation by leaf veins. Concentric rings of raised and
depressed necrotic tissue usually, but not always, give lesions a
characteristic "target board" or "bullseye" appearance (Fig.
48A). Leaf tissue often becomes chlorotic around and among
lesions. As new lesions develop and older ones expand, the
entire leaf becomes chlorotic, later necrotic, and desiccates but
"j usually does not abscise (Plate 33). Damage to leaves is
/ /"considerably in excess of tissue actually destroyed by lesions,
suggesting that toxins cause leaf death some distance from the
site of infection. Advanced vine symptoms intergrade with those
-of Verticillium wilt and leaf scald associated with moisture stress
in irrigated potatoes.
" Tuber lesions are dark, sunken, circular to irregular in shape,
and often surrounded by a raised border of purplish to gun
metal color (Fig. 48B). The underlying flesh is dry, leathery to
k, "corky, and usually brown. Tissue in advarced decay is often
water-soaked and yellow to greenish yellow. Lesions can
increase in size during storage, and tubers can become shriveled
, ,._ in advanced cases. Early blight tuber lesions are not as prone to
invasion by secondary organisms as are many other tuber rots.
'
,-. . . :. - C Causal Organism
Ahernariasolani Sorauer (syn. Macrosporium solani Ellis &
Fig. 47. Powdery mildew: A, early symptoms; B, Erysiphe Martin) has conidia 15-19 X 150-300 pm with 9-11 transverse
cichoracearum conidiophore; C, mature conidia. septa and few, if any, longitudinal septa. Spores are usually
borne singly but may be catenulate. They are straight or slightly
bent, the body being ellipsoid to oblong and tapering gradually
blight, with leaves becoming almost black (Plate 32), necrotic to a long beak (Fig. 48C and D). Color varies from pale to light
and abscising, leaving a rosette of upper foliage. Eventually, tan to olive-brown. The beak is flexuous, pale, occasionally
general infection can occur and the entire plant may collapse branched, and 2.5-5.0 pm wide. Conidiophores occur singly or
and die. in small groups and are straight or flexuous, pale to olivebrown,
6-10 pm in diameter and up to 100 pm long.
Causal Organism Cultural characteristics vary widely. Most isolates grow well
Conidia of Erysiphe cichoracearum DC. ex Merat form in on artificial media; however, they sporulate sparingly unless the
chains on unbranched conidiophores, 7-13 X 36-50 pm (Fig. mycelium is wounded or irradiated or theyare cultured on a low
4713). Mature conidia (Fig. 47C) are oval to ellipsoid with nutrient medium. Colonies are spreading, hairy, and grayflattened
ends. 13-16 X 20-30 pm, and lack well-developed brown to black. Some isolates produce a yellowish red pigment
fibrosin bodies when mounted in water or 10,7 KOH. in nutrient media.
Cleistothecia have simple appendages and contain several asci,
each usually containing two ascospores. Cleistothecia 135-165 Disease Cycle
/1i1 in diameter with indeterminate appendages and 5-10 asci Depending upon the location, A. solani persists in crop
are very rare, having been reported only on field-grown potatoes debris, soil, infected tubers, or other solanaceous hosts. The
from the western United States. fungus penetrates the leaves directly through the epidermis.
Differences in iesistance exist among tuber-bearing Solantm Primary infection can occuron older foliage early in the season.
spp. and within S. tuheroxtn. However, actively growing young tissue and plants heavily
fertilized with nitrogen do not exhibit symptoms, and most
Control secondary spread occurs as plants age, especially after bios-
I) )ust or spray foliage with elemental sulfur at intervals of soming, when secondary inoculutn levels are higher. In many
one to two weeks. locations, early blight is principally a disease of senescing plants.
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