NC State and USDA Cucumber Disease Handbook
Cucumber Disease Descriptions
General
Consult your local agricultural Extension Service for the latest
recommendations on the use of chemicals. The use of resistant
varieties; seed grown in the dry inner valleys of California;
prompt destruction of old cucurbit crops, volunteers, and certain
weeds; and practicing at least two years, preferably three, rotation
between cucumber, watermelon and cantaloupe will avoid many of
the disease problems. Viruses survive between cucumber crops on
weeds, other crops, and other cucurbits (squash, watermelon, cantaloupe).
They are spread from these hosts and between cucumber plants by
aphids and beetles. Powdery mildew also survives on similar plants
and is spread by wind currents. Scab, anthracnose, gummy stem
blight and angular leaf spot survive on infested plant residues,
on old plantings and volunteer cucurbits. All except scab may
be seed borne. Scab may spread in dew whereas the others require
rain to splash or blow the spores. Downy mildew survives on infested
plants in subtropical areas and spreads northward in storm currents.
Additionally all these diseases may be spread by machinery, personnel,
animals or insects moving through the fields, especially when
the foliage is wet.
Angular Leaf Spot
Angular Leaf Spot appears on leaves as small water-soaked spots
whose increase in size is limited by veins to form an angular
shape. The upper surface of the spot becomes tan, and a sticky
milky bacterial ooze forms on the lower surface and dries into
a white crust. Many of the diseased spots loosen and drop out
leaving a shot-hole effect. On stems and fruits the water-soaked
areas may also become covered with crusty white exudate. As fruits
mature, brown lesions form in the flesh and continue as streaks
down to the seed. Is infrequent and is caused by the seed-borne
bacterium Pseudomonas syringae pv. lachrymans which can also affect
squash and pumpkins. Warm, wet weather may cause stem and leaf
spots in mid-season. CONTROL: Follow general management practices;
start spray schedule at first appearance; do not enter fields
when plants are wet.
Anthracnose
Anthracnose is common and is caused by the seed-borne fungus
Colletotrichum orbiculare. Most cucurbits are susceptible. Most
commercial cucumber varieties have some resistance. Depending
on weather, leaf and fruit spotting may occur on young plants,
especially in late plantings. CONTROL: Follow general management
practices; start spray schedule at first appearance. Infects young
tissue and begins on a leaf vein as small yellowish circular spots.
Very young leaves may be distorted. The spots gradually become
brown and enlarge to 1/4 inch. They remain circular and the center
frequently drops out leaving a shot-hole appearance. Stem lesions
are shallow, elongate, slightly sunken, and tan in color. Fruit
lesions begin as small, circular, water-soaked spots which enlarge
to 1/2 inch and become tan. Under moist conditions pinkish spore
masses cover these spots.
Bacterial Wilt
Bacterial Wilt is caused by the bacterium Erwinia tracheiphila.
It overwinters in and is spread by cucumber beetles. Often seen
in early to mid-season cucumbers and when temperatures are cool,
it occurs on muskmelon and to a lesser extent on squash and pumpkin;
watermelons may be immune. CONTROL: Control the cucumber beetles,
spray.
Belly Rot
Belly Rot is caused by the fungus Rhizoctonia solani, present
in agricultural soils. It causes damping-off of many crops including
cucumbers. Belly rot is common, but is occurrence and intensity
are unpredictable. CONTROL: Avoid problem fields; apply fungicide
to soil prior to fruit set; keep harvested cucumbers cool, well
ventilated; use chlorine in wash.
Cottony Leak
Cottony Leak is caused by the fungus Pythium aphanidermatum and
others. Present in agricultural soils; it causes damping-off of
many crops including cucumbers. Cottony leak is common, but its
occurrence and intensity are unpredictable. CONTROL: Avoid problem
fields; apply fungicide sprays to soil and foliage prior to fruit
set; keep harvested cucumbers cool, well ventilated; use chlorine
in wash.
Downy Mildew
Downy Mildew is caused by the air-borne fungus Pseudoperonospora
cubensis. Although serious on late season cantaloupes, pumpkins,
squash, watermelon, and other cucurbits, it is seldom a problem
on cucumbers because most commercial varieties have adequate resistance.
The fungus over-winters in Florida, and each year the spores are
blown northward and arrive in North Carolina from May to August,
if at all. Typically its arrival coincides with summer rains.
CONTROL: Resistant cultivars; start spray schedule at first appearance.
Begins as small yellowish areas on the leaves. Early in the morning
when moisture has been present overnight, young spots may appear
as water-soaked, somewhat angular areas on the lower side. As
the lesions enlarge in a somewhat angular manner they change from
yellow to brown. On the lower side, purplish spore masses may
be seen, and several infected leaves may die in 10-14 days. Downy
mildew does not attack stems or fruit.
Gummy Stem Blight
Gummy Stem Blight is a warm weather southern disease. On leaves
it causes conspicuous V-shaped brownish lesions. Many times it
produces brownish circular spots which rapidly expand into large
tan lesions of round, indefinite size. On stems, circular fawn
(tan) lesions frequently elongate and turn gray and are present
mainly in the nodes. The lesions may exhibit brown gummy exudate
on the surface and stems may be girdled. In prolonged rainy periods
most of the foliage and vine may rapidly collapse. Fruit lesions
appear as small water-soaked areas, similar to anthracnose, which
rapidly enlarge to indefinite size. Also, a brown streak may appear
in the blossom end of the fruit. Although this disease may attack
young tissue, it is primarily a disease of older tissue. Is common
and is caused by the seed-borne fungus Didymella bryoniae. All
cucurbits are susceptible, and depending on the weather, stem
and leafspotting may occur on young plants, especially in late
plantings. Badly infected plants may have a gummy exudate from
stem lesions. CONTROL: Follow general management practices; start
spray schedule at first appearance.
Mosaic
The yellow and green mosaic patterns of cucumber viruses, commonly
seen on young leaves near the growing tip, often change to an
indistinguishable mottle as the leaves mature. A dwarfing of the
vines may become more conspicuous as leaf mottle symptoms tend
to disappear. Cucumber mosaic and watermelon mosaic viruses attack
cucumbers and produce similar symptoms; however, cucumber mosaic
virus does product a distinguishing symptom on fruit. Fruit infected
with cucumber mosaic are characterized by a lack of color ("white
pickle") in addition to the mottling and an extensive, bumpy,
roughened appearance produced by all the cucurbit viruses. Mosaics
are caused by viruses. Depicted is Tomato Ringspot, fairly common
in the northwestern part of the state. The virus resides in wild
brambles and is transmitted by some nematodes. Most commercial
cultivars have excellent resistance to Cucumber and Watermelon
Mosaic. Until recently mosaics were rarely seen in the eastern
counties. Because Zucchini Yellow Mosaic (closely related to Watermelon
Mosaic) may be introduced, it is important to identify all mosaics.
Mosaic viruses are transmitted by aphids and related insects.
CONTROL: Select resistant cultivars; avoid problem fields; start
spray schedule with stylet oil and insecticides at the seedling
stage; use aluminum plastic mulch.
Powdery Mildew
Powdery Mildew is caused by two air-borne fungi Erysiphe cichoracearum
and Sphaerotheca fuliginea, but is rarely seen since most commercial
cucumbers have excellent resistance. Powdery mildew is often serious
in dry weather and is the main cause of early dying of cantaloupes,
pumpkins, squash, and greenhouse cucumbers. CONTROL: Resistant
cultivars; sprays; because of fungicide specificity and possible
development of resistance in the fungus, switch fungicides if
spraying is not effective. First occurs on stems and leaves as
tiny, while superficial sports, becoming covered with powdery
spore masses as they enlarge. Under ideal conditions rapid, early,
shedding of leaves may occur as entire leaf surface becomes covered
with the powdery spores. Fruit is not infected.
Pythium Rot
This common rot starts as a small water-soaked spot, particularly
where fruit is touching the ground, which enlarges into a raised,
water filled blister about 1/4 inch in diameter. Further enlargement
is very rapid, with the disappearance of the blister symptom coupled
with the occurrence of soft, watery, brown rotten tissue covered
with a fluffy white mat. This entire process may occur in a 48
hour period.
Root-Knot
Root-Knot is caused by Meloidogyne spp. These nematodes are usually
present in agricultural fields and affect many crops. They are
especially damaging on plants growing in light, sandy soils when
counts are high. CONTROL: Promptly disk all crops after harvest,
treat soil with a fumigant nematicide when warranted by assay
report.
Scab (Spot Rot)
Scab (Spot Rot): first appears on leaves as small, circular to
angular water-soaked brownish spots difficult to distinguish from
angular leaf spot. Scab lesions are not covered with a crusty
white exudate. Under high humidity leaf sporulation occurs on
these spots as an olivaceous or velvety mat. On young fruit a
gummy brown exudate occurs on the surface of the water-soaked
areas, and as the spot spreads, the tissue sinks and dries to
form a tan scab. Brown streaks do not extend into the flesh as
for angular leaf spot. Is caused by the seed-borne fungus Cladosporium
cucumerinum which also affects muskmelons. It is rare because
most commercial cucumbers have excellent resistance. Cool night
temperature and high humidity is required. In the active phase,
dark, olive-green, velvety mold covers the spot. Leaves, fruit,
and young terminals may be affected. CONTROL: Use resistant varieties,
spray at first appearance.
Target Spot
Target Spot is caused by the air-borne fungus Corynespora cassiicola
which attacks many different plants. It is more common on cucumbers
in greenhouses than in fields. CONTROL: Follow general management
practices; start spray schedule at first appearance.
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