Common Crop Diseases and Their Control

Common Diseases of
Cucurbit Vegetables,
Mango, Corn and Onion
Agricultural Extension 217 –
Advanced Crop Protection

Mr. Pedro Ramos
Professor

Ronel D. Cana
Student, MSA

Introduction

Crop
Production
production of
involves in
food and other
depth plant
aim related
sciences
products and
and up-to-date commodities
technologies

Introduction

Crop
Protection
application of different technologies for the
purpose of increasing yield and minimizing, if
not totally eradicating yield loss

engages both the recognition
and control of plant pests and
diseases

Introduction

Tropical
Agricultural
Crops
among the crops that are most susceptible to
plant diseases
these susceptible crops include cucurbit
vegetables, mango, corn and onion

Diseases of Cucurbit Vegetables
Alternaria leaf spot
Cause: Fungus – Alternaria cucumerina and
Alternaria alternata
Symptoms: Small spots develop on the upper
surface of leaves which could develop into
larger coalescing lesions. These lesions might
have concentric rings.
Control: Use of disease-free seeds; Use of
recommended fungicides.

Alternaria leaf spot

Downy Mildew
Cause: Fungus – Pseudoperenospora cubensis
Symptoms: Leaves have a mottled
appearance. Leaf spots turn a pale yellow
color, enlarge and dry out. Can be confused
with the bacterial disease angular leaf spot.
Control: Use tolerant varieties where available; Use
a fungicide program that allows for the rotation of
protective and systemic fungicides which reduce the
chance of fungicide resistance developing.

Downy Mildew

Powdery mildew
Cause: Fungus – Podosphaera xanthi
Symptoms: White powdery spots develop on
leaves. Symptoms usually develop first on the
underside of leaves before covering both
sides. Leaves gradually turn yellow to a
papery brown and eventually die.

Powdery mildew
Control: Use tolerant varieties where
available; Monitor crops closely; Use a
fungicide program involving the rotation of
protective and systemic fungicides that
reduce the chance of fungicide resistance
developing; Petroleum spray oils, milk and
bicarbonates might offer some control.

Powdery mildew

Anthracnose
Cause: Fungi – Colletotrichum orbiculare
Symptoms: Brown to black spots develop on
leaves; long dark spots develop on stems and
round sunken spots develop on fruit. Fruit
symptoms might develop in transit.

Anthracnose
Control: Use tolerant varieties where
available; Monitor crops closely to identify
the disease early in its cycle; Use
recommended fungicides; Don’t replant in
sites that have previously had the disease, or
use a long rotation.

Anthracnose

Scab
Cause: Fungus – Cladosporium cucumerinum
Symptoms: Can affect leaves, petioles, stems and
fruits. Water-soaked spots occur on leaves and
runners. These spots eventually turn grey to white.
Lesions on fruit are often confused with
anthracnose. These spots are 3–4 mm in diameter
and might ooze a gummy substance. The spots
could then be invaded by secondary rotting bacteria
which cause the spots to smell.

Scab
Control: Use resistant cultivars; Use disease-
free seed; Apply appropriate fungicides
particularly if cool, wet weather is expected;
Rotate cucurbits with other crops, with two
or more years between cucurbits; Avoid low-
lying, shaded areas prone to moisture and
dews.

Scab

Damping-off of seedlings and fungal root rots
Cause: Fungi including Pythium, Rhizoctonia
and Fusarium
Symptoms: The first symptoms are water-
soaked lesions occurring at soil level. This
leads to wilting and seedling death. Often,
plants that have survived damping-off might
show symptoms of root rot. Roots can have a
watery grey appearance, particularly the finer
feeder roots.

Control: Usually sporadic outbreaks and can
be difficult to control; Fungicide seed
treatments provide some control of damping-
off; Plant to allow vigorous growth, and do
not overwater; Ensure residue from previous
crops is well-decomposed.

Bacterial leaf spot
Cause: Bacterium – Xanthomonas campestris
pv. Cucurbitae
Symptoms: Spots first appear leaves as small
water-soaked or greasy areas on the
underside of leaves, and as indefinite yellow
areas on the upper side of leaves. In about
five days the spots become round to angular
with thin, brown, translucent centres and a
wide, yellow halo.

Bacterial leaf spot
The spots enlarge up to about 7 mm
diameter. Occasionally young stems and
petioles are attacked. Young fruit may also be
affected. Fruit may produce a light brown
ooze from small, water-soaked areas, which
can extend into the seed cavity, causing seed
infection.

Bacterial leaf spot
Control: Use disease-free seed; Do not work
in the crop when it is wet; Practise rotation
and destroy plant debris by burning, if
possible, or ploughing in; Use recommended
chemical treatments, but often it is too late
after fruit symptoms are noticed.

Bacterial leaf spot

Mosaic
Cause: Watermelon mosaic virus, papaya
ringspot virus and tomato mosaic virus.
Symptoms: Light and dark green mottling of
the leaves. Distortion of leaves and stunting
of the plant might occur. Squash fruit might
show sunken concentric circles or a raised
marbled pattern.

Mosaic
Control: Use tolerant varieties; If only a few
plants are affected they should be removed
and destroyed; Insect control (Aphids) is an
option if monitoring is practised and
appropriate insecticide applications are
made; Plant residue should be incorporated
as soon as possible after harvest.

Mosaic

Common Diseases of Mango
Anthracnose
Cause: Glomerella cingulata, Colletotrichum
gloeosporiodies, Gloeosporium mangiferae.
Symptom: The disease appears on young
leaves, stem, inflorescence and fruits. Leaves
show oval or irregular grayish, brown spots
which may coalesce to cover larger area of
the leaves.

Anthracnose
Consequently, the development of the
affected leaves is hampered and they dry and
shed off. On young stem, grey brown spots
develop. These enlarge and cause girdling
and drying of affected area. Often, black
necrotic I areas develop on the twigs from the
tip downwards causing a dieback.

Anthracnose
Control
a. The fungus survives on dead and dried twigs,
therefore, they should be pruned/cut and burnt
as soon as possible.
b. The disease can be controlled by spraying
Bordeaux I mixture (3:3:50) Blitox or Phytolan
(0.3%), Bavistin or Benlate (0.1 %), thrice a year..
c. Affected full grown fruits should be dipped in
hot water (51OC) for 15 minutes before storage.

Anthracnose

Powdery mildew
Cause: Odium mangiferae

Symptom: A white superficial powdery mass
of fungus growth covers the leaves, the stalks
of the panicles, the flowers and tender fruits.
The powdery mass consists mainly of fungal
spores. The mildew affects the flowers before
fertilization and the fruits in their earliest
stages.

Powdery mildew
The affected flowers and fruits drop
prematurely, reducing the crop considerably
or preventing the fruit set. Rain and mist
accompanied by cool nights during the
flowering period, favors the development of
the disease. Shedding of infected leaves may
also occur.

Powdery mildew
Control
a. The most effective method of control is by
dusting finely powdered sulfur on the flowering
branches. More than one round of dusting will
be necessary. The first round should be given
before flower opening. This will be followed by
two or more rounds at fortnightly interval.
b. It can also be controlled by the application of
Kemthane (0.1%) or Bavistin (0.1%) or Benlate
«0.1%). The spray should be given before the
flowering, as a preventive measure.

Powdery mildew

Bacterial canker
Cause: Pseudomonas mangiferae

Symptom: The earliest symptom is the
formation of small dark green water soaked
spots which finally assume the shape of a
raised black spot. These areas on the fruits
develop longitudinal cracks and gum starts
oozing out from the splits.

Bacterial canker
Seriously affected fruits drop down and the
yield is reduced since affected fruits are
unattractive and unmarketable.

Bacterial canker
Control
a. Bordeaux mixture (4:4:50) may be applied at
fortnightly interval from the first appearance of
the disease on the leaves.
b. Recently, two sprays of Streptocycline (200
ppm) have been found to be helpful in reducing
the incidence of this disease.
c. Field sanitation and seedling certification as
preventive measures and seedling branches and
fruits are reported to minimize the infestation of
this disease.

Bacterial canker

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hmuan mnid. Aoccdrnig to a rscheearch at
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oredr the ltteers in a wrod are, the olny iprmoatnt
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Common Diseases of Corn
Seed Rot, Seedling Blight, Root Rot
Cause: Pythium, Fusarium, Gibberella,
Trichoderma and Penicillium
Symptoms: The color of the roots most often
provides a good indication of which organism(s) are
present:
• greyish-white indicates Diplodia
• tan to pink indicates Fusarium or Gibberella
• reddish to brown indicates Rhizoctonia
• blue-green indicates Penicillium or Trichoderma

Symptom: Seed, seedling and roots infected
by Pythium are most often soft (wet) and
dark coloured, as opposed to roots infected
with Fusarium, Gibberella, Diplodia and
Rhizoctonia, which are firm or leathery.

Anthracnose Leaf Blight
Cause: Colletotrichum graminicola

Symptoms: Anthracnose may affect both
leaves and stalks. The main symptoms are
leaf spotting, top dieback and stalk rot. Leaf
spots are oval, up to 15 mm (6 in.) long, with
a tan centre and reddish-brown border.
Individual lesions may join, forming streaking
along the margin or midrib.

Symptom: A general yellowing of the tissue
surrounding the infected areas often
develops. With the aid of a hand lens, small
black spots can be seen in the centre of the
lesions. The disease is first observed on the
lower leaves and later on the upper leaves.
Top dieback can occur late in the season as
diseased leaves wilt and gradually die.

Control
a. Planting resistant varieties
b. Practicing crop rotation
c. Using of effective fungicides

Common Smut and Head Smut
Cause: Ustilago zeae and Sporisorium holci-sorghi

Symptoms: Greyish smut galls up to 10 cm (4
in.) in diameter develop on the stalks, ears
and tassels, while smaller galls often appear
on the leaves. The galls initially have a white
membrane cover that eventually breaks and
releases dark-brown or black powdery
spores.

Symptom: On the leaves, galls develop into a
hard, dry growth. Smut galls can replace
kernels. Unlike common smut, head smut
occurs only on the ears and tassels.

Control
a. Minimizing mechanical and chemical injury to
the plant;
b. Practicing a balanced-fertility program
c. Using of effective fungicides

Anthracnose Stalk Rot
Cause: Colletotrichum graminicola

Symptoms: Large, dark brown-to-black shiny
areas or streaks on the outer stalk rind. These
shiny or discoloured areas are often found at
the base of the stalk. Cutting the stalk
lengthwise will reveal a discolored and rotted
pith. Premature death occurs above the ear,
with the plant tissue below the ear remaining
green.

Examination of the stalk in these dead areas
will show the same shiny black areas that are
found at the stalk base. Plants with top
dieback symptoms correspond to areas of the
field that had late-season stresses.

Control
a. planting hybrids that have good resistance or
tolerance to leaf diseases and stalk rots
b. managing insects such as corn borer
c. good weed control
d. appropriate plant populations
e. a balanced N and K fertility program
f. crop rotation

Fusarium Ear Rot
Cause: Fusarium verticilloides

Symptoms: Unlike gibberella ear rot, kernels
infected with fusarium ear rot will be
scattered around the cob among healthy-
looking kernels or on kernels that have been
damaged (by corn borer or bird feeding). Silks
are susceptible to infection during the first 5
days after initiation.

Fusarium Ear Rot
Fusarium infection produces a white-to-pink
or salmon-coloured mould. A "white
streaking" or "star-bursting" can be seen on
the infected kernel surface.

Fusarium Ear Rot
Control
a. Harvest as early as possible.
b. If bird damage is evident, harvest outside
damaged rows separately. Keep and handle the
grain from these rows separately.
c. Adjust harvest equipment to minimize damage
to corn.
d. Cool the grain after drying.
e. Clean bins before storing new grain.

Fusarium Ear Rot

Common Diseases of Onion
Purple Blotch
Cause: Alternaria porri

Symptoms: Small white sunken spots develop
on the leaves. These enlarge and under moist
conditions, turn purple. These are also
prominent on the inflorescence stalks.
Infection can cause a semi-watery rot on
necks of bulbs that turn yellow-red in color.
Infected bulb tissues become papery.

Purple Blotch
Control
Cultural control methods include long rotations
with unrelated crops and good drainage. Lowering
the density of transplanted crops will reduce
infection, as will the application of high rates of
calcium superphosphate and potassium fertilizer.
Nitrogen fertilizer at low and high rates will
increase the prevalence of disease. Routine
(weekly interval) field sprays with dithiocarbamate
fungicides, particularly mancozeb and
chlorothalonil.

Purple Blotch

Anthracnose
Cause: Colletotrichum gloeosporioides

Symptoms: The leaves become twisted due
to infection.

Control
Control measures are similar to purple blotch.

Anthracnose

Downy Mildew
Cause: Peronospora destructor

Symptoms: Appearing as white specks,
usually confined to the oldest leaves of young
plants. A white mold develops rapidly in cool
damp weather and progresses down the
sheath, and plants eventually fall over and
dry up.

Downy Mildew
Control
Young plants can be treated with mancozeb at
weekly intervals until bulbing begins.

Downy Mildew

Basal Rots
Cause: Fusarium spp.

Symptoms: Outwardly, the bulb may appear
normal. It eventually becomes soft, however,
and will develop a watery rot under moist
conditions or a dry shriveled bulb in a dry
environment.

Basal Rots
Control
Application of fungicides, good field drainage and
removal of excess moisture in the storage area.

Basal Rots

That’s it...

Oops! There’s one more:

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Common Crop Diseases and Their Control

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