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

penetration, and subsequent disease development. Because
protectant fungicides are not absorbed and translocated
through the plant to any significant degree, they must be applied
uniformly to as much of the foliage as possible. This implies
uniformity both in horizontal distribution of fungicide across
the spray swath and vertical distribution (penetration) through
the plant canopy. Coverage of both upper and lower leaf
surfaces isalso essential. To achieve optimal coverage, attention
should be paid to the following: use of the correct spray volume
and pressure for which a sprayer has been designed; proper
functioning of the sprayer as determined by cleanliness, wear of
nozzle orifices, height of boom, accuracy of pressure gauge;
spraying when the air is still; and not extending swath width
beyond that specified for the sprayer.
Traiditional hydraulic boom sprayers that apply large
voluimes (70t 1170r L ha, 75-125 g A of dilute spray under
high pressure (approximately 28 kg cm, 400 psi) provide good
horizontal and vertical distribution of fungicides. However,
weight, high cost, large water requirement, and frequent need
for filling have decreased their popularity in recent years. The
present, trend is towards low pressure (approximately 7.0-8.8
kcn)-15pilwvlm(apoiaey45Lh,0
kg cm -100-125 psi), low volume (approximately4651 ha,50
g,A or less) spraying, either with hydraulic or airblast
(atomizing) machines. These can be effectively used if the
manufacturer's recommendations are followed,
Spraying by aircraft has several advantages such as speed,
saving of labor, ability to be used in fields too wet for ground
equipment, absence of yield loss due to sprayer tracks, reduced
spread of virus diseases, and low water requirements (usually
28-47 1. ha, 3-5 g A). These advantages have increased accetance
of aircraft application particularly among large growers,
But again, certain disadvantages must be considered, such as
wind iterference and risk of drift, unsuitability of small,
irregularly shaped or topographically uneven fields, the hazard
of physical obstacles, relatively poor vertical distribution of
spray through the plant canopy, and sparse deposition of
d3scr,;et particles of concentrated fungicide. This last makes it
essential that fungicide deposits be redistributed, i.e.,
transferred from their original landing sites to unprotected leaf
areas by moisture in the form of dew, rain, or irrigation.
Although the addition ofadjuvantsto fungicide sprays has been
promoted to improve redistribution, most fungicide
formulations have adequate spreadability without being
supplemented with such materials.
To provide as complete a protective blanket as possible,
fungicide sprays must be applied at regular intervals to protect
newly-expanded foliage and to supplement fungicide activity
lost to dilution, photodegradation, oxidation, etc. Frequency of
application depends n weather conditions, presence of disease
in the vicinity, tena,:ity of the fungicide, varietal resistance,
sprayer capability, and other factors. The usual interval
employed by growers ranges from 5to 14 days. In certain areas,
disease forecasting systems are used to determine spray
intervals,
I se ofthe correct aimount offungicide per unit area isvital to
effective disease control. This requires careful attention to
spr,.ycr calibration and maintenance. Calibration means determining
how much fungicide is applied by a sprayer to a particular
area.
In summary, the four basic requirements for success are to
apply the right chemical in the right amount at the right time in
the right way to obtain maximum coverage.
Selected References
IIJRSFA I.I. .1.G. 1956. Principles of Fungicidal Action. Chronica
Botanica Co., Waltham, ME. 231 pp.
MARlIN. II. 1959. The Scientific Principles of Crop Protection. 4th
ed. EIdward Arnold, Ltd.. London. 359 pp.
10RGESON,1). C.. ed. 1967. hIngicides An Advanced Treatise, Vol.
I. Academic Press. New York. 697 pp.
(Prepared by 0. Schultz)
Tuber Seed Treatment
Chemical treatment of seed tubers before planting is neithera
cure-all nor a replacement for the use of high quality seed,
properly stored and handled. Rather, it isinexpensive insurance
that partially protects seed from invasion by microorganisms
present in the soil and on the tuber surface. Unfortunately, it
does not kill microorganisms present within seed.
Possible benefits of seed treatment are: I) control of storage
diseases such as Fusarium dry rot, 2)control of seed piece decay
when cut seed has to be held for an extended period before
planting. 3) control the soil (usually
ol + seed piece decay inthsoluuay
caused by Fusarium spp.) when planting is done under adverse
soil conditions (cool and wet) that impair suberization of the cut
surfaces, and 4) partial control of diseases such as Rhizoctonia
stem canker, scab, Verticillium wilt, and blackleg.
Prestorage treatment of seed, a relatively recent development,
usually involves application of fungicides (e.g., thiabendazole)
in the form of a fine mist as tubers enter the storage structure.
Seed treatment after storage and before planting is most
commonly don: by dusting with rotating drum dusters, which
generally provide better coverage than do the shaker types of
treaters. Several ethylenebisdithiocarbamate fungicides are
useful for dip treatment of seed tubers. but they are not com­
monly applied in tis manner.
When whole seed is planted, or when cut seed is planted in a
warm, moist seedbed promptly after cutting, whole tubers may
be surface disinfested by dusting or dipping. This is only
partially effective in controlling diseases such as Rhizoctonia
stem canker, scab, and Verticillium wilt because their causal
agents are commonly soilborne. Where cut seed is
predominantly used, treating seed immediately after, rather
than before, cutting is advisable (dusts. are preferred). This
protects the cut tuber surfaces from invasion by soilborne
pathogens such as Fusarium spp. and provides partial surface
disinfestation.
Cut seed, whether treated or not, should be planted
immediately. If this is impossible, it should he stored in open
containers such as potato crates stacked to provide ample
ventilation. Burlap bags should be only half filled and also
stacked forventilation. Cut seed should be held at 10-16Cand
high relative humidity (85-90%) for three to four days to allow
wound cork to form over cut surfaces. If further stoage is
necessary, temperatures can be dropped to 5 C, but seed should
be warmed again before planting to ensure vigorous sprouting.
Insects should be excluded from cut seed, particularly adults of
the seed corn maggot. Cut seed held for some time before
planting may develop considerable seed piece decay and give
poor stands.
Selected References
BOYI). A. E. W. 1975. Fungicides for potato tubers. Proc. 8th British
Insecticide and Fungicide Conf. 3:1035-1044.
MOSHER, P. 1972. Treating seed potatoes for disease control.
Spudlines. April 1972. Univ. of Maine. 3 pp.
(Prepared by 0. Schultz)
67