FOLIAR DISEASES
In this section, "foliar disease"
will be limited to a symptom that not only appears on the foliage, but started
there. Thus, diseases where leaves are wilted because of root rot or clogging
of the vascular system will not be discussed here. In diagnosing diseases,
it is therefore important to examine all parts of the plant, including ones
far removed from the obvious symptom. There are many foliar diseases caused
by microorganisms, but this article will only treat the major ones in south
Texas.
Cercospora leaf spot is caused by the fungus, Cercospora citrullina, and is the most prevalent foliar disease of watermelon in south Texas, but it is not usually a serious problem. It has a characteristic symptom: circular brown spots with white or tan centers. Sometimes the area surrounding the brown spot is slightly chlorotic. Heavy infection leads to defoliation and therefore chemical control is advisable. The disease is favored by wet conditions and warm temperatures of 80-90F. The literature says that this pathogen overwinters on crop debris and cucurbit weeds, yet I have seen the disease on new watermelon land and have noticed only healthy-looking citron melons and burgherkins in the midst of symptomatic watermelon plants. Airborne movement of spores is important for introducing the pathogen into fields. The disease can be managed with a spray program of protectant fungicides such as coppers (e.g. Kocide), mancozeb (Dithane, Penncozeb). maneb, and chlorothalonil (Bravo).
Downy mildew is caused by the fungus
Pseudoperonospora cubensis, does not reliably appear in fields, but
its occurrence should always be a cause for concern because of its ability
to quickly increase and become uncontrollable. It is first seen as irregular, yellow areas on the upper leaf surface.
The centers of the areas become brown and they
are surrounded by a yellow area . Under conditions of high humidity,
fuzzy, grayish areas are seen on the lower
leaf surface. It is best to make the examination for this early in the
morning. Alternatively, suspect leaves can be removed and kept several hours
in a plastic bag with a moist paper towel to provide high humidity that
encourages sporulation. As the disease progresses, the entire leaf dies
and the foliage in a field where this disease is running rampant look like it was burned. The disease is favored
by cool (60F night and up to 77F day) temperatures along with rain or continuous
high humidity. This disease can occur in a field miles away from other cucurbits,
indicating that the spores can travel great distances. Excellent control
of this disease can be achieved with mefanoxam (Ridomil Gold), which must
always be used simultaneously with a protectant fungicide such as mancozeb
or chlorothalonil to prevent the emergence of resistant strains. Furthermore,
these sprays must be applied on a preventative basis, before the disease
becomes established in a field. Some growers report that they only use chlorothalonil
or mancozeb to effectively manage downy mildew, but, again, the key to their
effectiveness is regular, preventative applications.
Alternaria leaf blight is caused
by the fungus Alternaria cucumerina, is not as much a problem on
watermelon as it is on cantaloupe. The symptom is a large, dead lesion on
the leaf. This symptom could easily be confused with similar symptoms produced
by insects or physical damage. Furthermore, leaf tissue killed by other
things can be colonized by other, saprophytic Alternaria species.
The disease is favored by continuous wet conditions, but it can be controlled
by the same fungicides that control Cercospora leaf spot.
Gummy stem blight is caused by the
fungus Didymella bryoniae, causes large, circular dark brown spots
on leaves. Later, small black, pimple-like structures appear in these spots.
This is an infrequently seen, minor disease of watermelons in south Texas
and it's a good thing, too, because the fungicide of choice, benomyl, can
not be used in this area because of the presence of resistant strains of
the fungus. This is an important disease of watermelon in the northern part
of the state and in other states, just like another not-seen-in-south-Texas
fungal disease known as anthracnose. Evidently, south Texas is not humid
enough for these diseases to thrive on foliage.
Powdery mildew will normally be
seen as a white, powdery residue primarily on the upper leaf surface, is
caused by two species of fungi, Sphaerotheca fuliginea and Erysiphe
cichoracearum. The former species is prevalent in south Texas (the two
species can be differentiated by microscopic examination). This disease
is favored by high humidity, but not free water on leaves. It tends to occur
on older leaves and later in the growing season, so control is not usually
warranted. Two systemic fungicides, Benlate (benomyl) and Bayleton (triadimefon),
are no longer effective because of the presence of resistant strains. There
are two types of protectant fungicides labeled for powdery mildew, namely,
coppers (e.g. Kocide) and sulfur (e.g. Copper/Sulfur F). These fungicides
are not very effective under south Texas conditions.
Fruit blotch is caused by the bacterial
pathogen, Acidovorax avenae subsp. citrulli, can also cause
foliar symptoms. Curiously, where I have seen abundant fruit blotch in south
Texas fields, I have seen no foliar symptoms on the affected plants. Yet,
the only time I've seen abundant leaf symptoms of this pathogen, there were
no fruit symptoms. Other watermelon growing areas report both leaf and fruit
symptoms, so it is likely related to weather conditions (although the cultivar
or other factors may affect symptom development). The pathogen can cause
large, circular brown lesions, usually near the edge of the leaf, but this
is not reliable for diagnosis. The unique symptoms consist of irregular,
rust-colored lesions that run along leaf ribs. On the underside
of the leaf, the leaf rib is water-soaked. Leaf lesions can serve as
a source of bacteria that can infect fruit, but I suspect that in most cases,
by the time symptoms are seen on the leaves, it is too late to prevent fruit
infection. This is why it is prudent to use copper sprays on a preventative
basis.
Viruses usually result in no necrosis (death) of leaf tissue. The
squash leaf curl virus, transmitted by whiteflies, will cause an irregular yellowing of leaves associated with leaf
distortion. Leaves appear to be tightly
clustered around the vine. This is a common disease of fall-grown watermelons
and large yield losses can be avoided by using transplants, or direct-seeding
when there are low local populations of whiteflies. A green
mosaic symptom (i.e. light and dark areas of green), sometimes with
leaf deformation, can be caused by two aphid-transmitted viruses, the watermelon
mosaic virus and papaya ringspot virus W. These viruses are infrequent on
south Texas watermelons, usually limited to one or two plants in a field
and never spreading. These viruses can cause major losses in other production
areas. The tobacco ringspot virus, which is transmitted by the dagger nematode,
causes small, chlorotic or necrotic spots,
or ringspots on leaves. It is not an important disease, except when it produces
fruit symptoms, for example, necrotic pimples,
or raised necrotic "target spots".
It can be a problem in weedy fields or land that was recently brought into
cultivation. Cultivation and weed control can reduce this disease because
they cause reductions of the nematode populations in soil.
This section only discusses infectious diseases. Other, non-living agents
can cause foliar symptoms and these can be more difficult to diagnose. However,
the first step to diagnosing these problems is to rule out the infectious
pathogens.
ROOT AND VASCULAR DISEASES
Diseases that affect the roots and vascular system of watermelon plants
cause wilting of vines and leaves. Thus, when attempting to diagnose the
cause of wilting, it is essential to examine the roots and crowns of affected
plants. The pathogens involved in these diseases are soilborne and, with
the exception of nematodes, are very difficult to manage. However, the effects
of these pathogens when they infect roots or stems are generally not as
devastating as diseases caused by foliar pathogens, except in soils where
the watermelon crop has been grown frequently and the pathogen has built
up to high levels. One of the reasons for this is that soilborne pathogens
are not mobile like foliar pathogens and so will not spread as rapidly throughout
the field. (Soilborne pathogens can be very devastating when they infect
fruit; this will be the subject of another article).
Damping-off is a disease that affects
young seedlings. Seedlings wilt and die,
or seeds may not emerge. Damping-off is caused by fungi: Rhizoctonia
solani or several species of Pythium. Generally, damping-off
operates best at low temperatures that are sub-optimal for germination and
growth of watermelon. However, I have documented the presence Pythium
aphanidermatum in several areas of south Texas. This particular species
grows optimally at high temperatures and I have seen it killing
newly-transplanted watermelons during a period of warm weather. Watermelon
seeds are usually treated with broad-spectrum contact fungicides that would
protect against all damping-off pathogens, but these are of limited value
once the seed germinates and starts growing. Nothing can be done if seedlings
are growing under conditions favorable for R. solani, but soil applications
of Ridomil Gold will provide protection against Pythium species.
Nematodes can substantially reduce
yield of watermelon. The affected plants are stunted and chlorotic. Root-knot
nematodes can cause galling on roots, but
there are no visible root symptoms with the reniform nematode. However,
I have never seen any evidence of nematode problems in watermelon production
in south Texas and for this reason, I recommend testing soils for the presence
of nematodes rather than routinely treating with nematicides. There is good
information about the distribution of these nematodes in the Lower Rio Grande
Valley, based on surveys conducted over a 22-year period. The root-knot
nematode occurs primarily in sandy soils and it has been well-documented
in the Rio Grande City area, as well as northern portions of Hidalgo county.
Sandy soils in the Premont and Falfurrias areas could support the root-knot
nematode, but I have not seen any problems in these areas. The reniform
nematode occurs in fine textured soils (high amounts of silt and clay),
predominantly in areas of the Valley where watermelons are not grown, although
this nematode has also been documented in the northern portion of Hidalgo
county. Although nematicides can provide an economical control, a crop rotation
with corn or sorghum and a weed-free fallow can reduce numbers of nematodes
below damaging thresholds.
Fusarium wilt is a soilborne disease caused by the fungus Fusarium oxysporum f. sp. niveum. The first symptom is a temporary wilt of vines, which occurs during the hottest part of the day. The wilt becomes permanent and progressive, affecting more vines. Eventually, the plant dies. If a cross-cut is made in the stem, a brown discoloration of the vascular system can be seen. The best place to see this discoloration is near the crown, especially if the vines are only slightly wilted. The roots are not affected. The disease is controlled by the use of resistant cultivars, however, there are no commercially-available varieties that are resistant to race 2 of this pathogen, which I have documented in some of the sandy soils of northern Hidalgo county and the Falfurrias-Premont area. This pathogen will persist for years in soil in the absence of watermelon. Rotations of 5-7 years are recommended, but even longer periods of time may be required to reduce pathogen populations. I have not seen this disease occurring in heavy, alkaline soils of south Texas.
Anasa wilt is caused by a toxin produced
by the squash bug and can be mistaken
for Fusarium wilt. This problem is associated with watermelon growing next
to brush and squash bugs can be seen on some of the plants.
Monosporascus vine decline is caused by a soilborne fungus, Monosporascus cannonballus. It seems to operate in the soils that Fusarium wilt does not occur in, namely, the heavy, alkaline soils in the Lower Rio Grande Valley. It is most prevalent in the Rio Grande City area, where fields have been cropped extensively with cantaloupe. It is less common in other areas of the Valley. It causes a sudden, permanent wilt of vines. Sometimes there is vascular discoloration, but this is not a reliable diagnostic feature. Discrete, dark brown lesions can be found on the roots. There are no control measures for this fungus.
Other pathogens may be involved in wilting, but their pathogenicity has
not been demonstrated and until further information is gathered, no recommendations
can be made. For example, Rhizoctonia
solani and a Pythium sp. were isolated from roots which had a light brown discoloration.
Southern blight is caused by the
fungus, Sclerotium rolfsii, which thrives under moist soil conditions
at high temperatures. I have seen this on one occasion, in mid-Hidalgo county,
affecting watermelon growing under drip irrigation and plastic mulch. The
symptoms are wilting and a white, prolific
cottony growth of the fungus in the crown area. The fungus later produces
brown, spherical resting bodies, which resemble mustard seeds. This is not
an important disease of vines, but under certain conditions, it can cause
a fruit rot (which could be minimized through the use of drip irrigation
and plastic mulch).
FRUIT DISEASES
The unfortunate thing about finding disease symptoms on fruit is that identification
of the problem can not lead to action that will provide a solution. It is
too late. In the case of bacterial fruit blotch, at least knowing the cause
can give growers the option to prevent future problems. But usually, with
many soilborne diseases, there is no expectation that fruit diseases can
prevented in the future. However, the appearance of many of these diseases
is dependent upon favorable (usually extreme) weather or growing conditions.
There may also be an interaction between these weather conditions and the
cultivar. So, a problem seen in one year may not be seen again.
Bacterial fruit blotch symptoms on the fruit start as water-soaked
spots that enlarge over time, but remain firm. The lesion has a dark, oily
appearance and may have cracks with a brown
gummy exudate. The infection does not cause decay of the fruit, but
the cracking of the rind can allow entry of secondary pathogens that can
cause fruit rot. Lesions are usually found
in the light portions of striped fruit and light fruit is more susceptible
to this disease than dark fruit. The disease may appear to develop overnight,
but actually, the pathogen was present all along on the foliage of plants.
Mature fruit will not become infected, so the disease will not spread to
other watermelons in storage. If this disease is present in a field, control
measures need to be taken to ensure that it does not become established.
These measures include at least a two year rotation away from watermelons
and other cucurbits, as well as a vigorous control of volunteer watermelons
and any cucurbit weeds in the field during this time.
Bacterial rind necrosis exhibits no external symptoms. When
the rind is cut open, brown areas are seen in the rind. These areas
are not rotted. The flesh is rarely affected. The circumstances leading
to the disease are not clear. Something triggers the growth of low populations
of bacterial that are naturally present in the rind, leading to the
necrosis. This can be a combination of cultivar and growing conditions.
The incidence of this problem can be very low in a field, but because affected
fruits can not be culled, the crop from a field may become unmarketable.
There are no control recommendations, although a grower should investigate
whether other cultivars are also affected in the same field.
Belly rot can be caused by several species of soilborne fungi. One
symptom is the appearance of a small, water-soaked spots that enlarge and
become soft. Often, a white fungal growth can be seen on the outside of
the fruit or even on the inside, where the fruit is decayed. Fruits infected
with Pythium aphanidermatum are soft
and have a dull brown appearance. Another species, Sclerotium rolfsii,
the southern blight fungus, produces a profuse, white growth on fruit, and
later, small, tan spherical survival structures
that resemble mustard seeds. Belly rot becomes a problem when soil remains
saturated for a prolonged period of time, particularly when temperatures
are high. If soil drainage problems can not be alleviated, then plants should
be grown on raised beds, particularly on plastic mulch.
Pimples: The symptoms are small bumps or pimples on fruit. There may also be ringspots (circles of darker coloration) or raised, brown, concentric rings. These symptoms can be caused by the tobacco ringspot virus. This virus is vectored by the dagger nematode. Other species of nematodes, such as the root knot nematode, do not vector this virus. (The presence of the dagger nematode in a field can not be determined by visual symptoms, but only by a soil test). High populations of this nematode have been found in weedy fields or in pasture land. Thus, this problem may occur on new watermelon land. The epidemiology of this virus on watermelons has not been studied. In South Carolina, the virus was found in smooth pigweed growing in a squash field and was found to be seed-transmitted in this weed. Back in the 1950's, this virus was documented on many weeds and crop plants in the Lower Rio Grande Valley. This virus has a similar appearance to the early symptoms of anthracnose, a fungal disease. (However, anthracnose has NOT been documented in south Texas; apparently, the climate is not favorable for this disease). With pimple disease, the pimples do not change in size and the fruit remains firm. With anthracnose, the pimples become concave, enlarge, and eventually affect the flesh, leading to fruit decay.
The control for tobacco ringspot virus is to aggressively control the weed
hosts that harbor the virus. Cultivation and fallowing of land will lead
to a decline of populations of the dagger nematode, so a nematicide treatment
may NOT be necessary, or even make a difference. If in doubt about the presence
of economically-damaging levels of this nematode, soil should be tested.
Physiological disorders: These physiological disorders are not common, particularly in fields that are properly maintained and have not been subjected to extreme weather conditions.
Blossom-end rot is caused by a calcium
deficiency in the fruit and occurs in plants subjected to drought stress.
The initial symptoms are brown spots at the blossom end of the fruit which
later enlarge, darken and become leathery. Decay may occur as a result of
microbial activity. Cultivars can differ in their susceptibility to the
problem. Watering operations that avoid drought stress can reduce or eliminate
the problem. Excess nitrogen fertilizer may also contribute to blossom-end
rot.
Bottle neck or otherwise misshapen fruit can be caused by pollination
problems or fluctuations in watering. Cultivars can differ in susceptibility.
Cracks, scarring and
pitting can be caused by mechanical damage
when fruits are young, Insects can also cause such injury.
Splitting is caused by an erratic watering pattern, particularly the excessive accumulation of water. The inside of the fruit is under great pressure and the fruit splits open when lightly tapped. Never, ever refer to this as "exploding", as this term is not an accurate or market-friendly description of the problem. This condition is weather related and there probably is a cultivar interaction as well.