When used as a verb, to rogue means to get rid of items that
don’t conform to a certain standard. In
plant pathology, the word rogue is used to describe a technique whereby
diseased plants are removed or rogued to slow the spread of disease. I’d like to describe the practice as it
might be used to manage Phytophthora blight of pumpkins.
The practice works like this: Under conducive conditions, Phytophthora
blight spreads quickly from leaf to leaf and from plant to plant. From a single diseased pumpkin plant, an
entire field can become infected. But
what if one could rogue the few symptomatic plants at an early stage in the
disease epidemic? Would this slow the
spread of Phytophthora blight?
In theory, yes. If
one were able to rogue all of the diseased plants in a field, the disease could
be slowed. It would be similar to
sending sick children home from a classroom;
the disease should progress at a slower rate with sick children removed
than if they had stayed and infected more children. However, in practice there are a few
complicating factors. Read below for
Pumpkin plants may become infected with the organism that
causes Phytophthora blight either by coming into direct contact with
soil which harbors the causal organism or from spores that are spread from
diseased plants. The practice of roguing
is designed to slow secondary or plant to plant spread of the disease. Phytophthora
blight that is caused by direct contact with the soil will remain
unaffected by roguing. Therefore,
roguing diseased plants will not stop new infections from soil borne fungi,
however, this practice should slow the secondary or plant to plant-spread of Phytophthora
Another complicating factor—It is almost impossible to
completely eliminate all diseased plants.
The reason is that pumpkin plants with Phytophthora blight do not
show symptoms immediately. There is a
period of 3 to 5 days between when the pumpkin plant is infected to when symptoms become visible (in plant pathology,
this is known as the latent period). So,
if one were to rogue all symptomatic plants, almost certainly some of the
adjacent plants are infected but not showing symptoms yet. The best solution to this problem is to rogue
some of the healthy plants along with the diseased ones. Or, as they taught us
in graduate school, rogue till it hurts.
If one must remove apparently healthy as well as diseased
plants when roguing, how many healthy plants must be rogued? Unfortunately, there is no mathematical
formula for estimating how many healthy plants to rogue. However, let's assume that a rain storm
accompanied by strong winds can blow splashed spores 10 to 15 feet. If most pumpkin plants are on 6 foot centers,
then one should remove about two rows of apparently healthy plants in addition
to the diseased plants. Each grower will
have to estimate the amount of healthy plants to rogue based in his or her own
Roguing for disease management is most likely to be
successful if attempted early in the disease epidemic. Let's imagine that a few pumpkin plants are
observed with Phytophthora blight in a low area of the field. The decision to rogue is made. The diseased plants are cultivated under as
well as 2 or 3 rows of healthy plants beyond the plants with symptoms. The cultivation equipment is cleaned off to
prevent soil from the diseased field from being carried to a different field. Such a situation is shown below in Figure
1. While success is not guaranteed,
roguing has the potential to slow disease spread.
A situation where roguing is less likely to be successful is
one where much secondary spread has already taken place. If a relatively large area of the pumpkin
field already has symptoms of Phytophthora blight, the disease may have
spread beyond where roguing may slow disease spread. If the field has a long history of Phytophthora blight
of the field, roguing may not help.
whether to conduct a roguing operation to manage Phytophthora blight of
pumpkin, it may help to know whether secondary spread of this disease has
occurred. Initial or primary spread of Phytophthora
blight usually occurs in low areas of the field. Since initial outbreaks of Phytophthora
blight are likely to come from fungi that have survived in the soil, the
first plants to be affected often have lesions where the plant has the most
contact with the soil, at the very base of the plant where the main stem meets
the soil. (Mature fruit which comes into contact with the soil may also have
symptoms of Phytophthora blight. By the time mature fruit are present
and symptomatic, however, secondary spread is likely to have occurred.) Secondary spread, that is, disease that has
occurred as a result of the splash of spores from a plant initially infected to
the leaves and stem of healthy plants often occurs on the leaves, petioles or
branches of the pumpkin plant.
other circumstances where roguing may be used as a disease management
tool. Each circumstance, however, must
be considered on its own merits. Please
let me know if you have any thoughts or questions.
Figure 1: In a field of pumpkins with Phytophthora blight, a portion of the field with symptomatic vines have been plowed down or rogued, to slow the spread of the disease.
The relatively cool weather Indiana has experienced this
summer may be responsible for more observations of northern corn leaf blight
(NCLB) on sweet corn than normal. The
primary symptom is the cigar shaped lesion that ranges from 1 to 7 inches in
length (see Figure 1). The lesions may range from tan to gray in color. Under
conditions of high humidity, olive-green fungal spores may be produced on the
lesion surface. Symptoms of NCLB are frequently observed late in the season
when days become cooler. Yield losses are possible if lesions reach the ear
leaf or higher during the two weeks before or after tasseling. NCLB can be
managed by a combination of crop rotation, fall tillage, resistant hybrids and
fungicide applications. Crop rotation and fall tillage help to minimize crop
residue that might harbor the fungus that causes NCLB. Choose hybrids resistant
to NCLB when possible. When it is necessary to use hybrids without resistance
and weather conditions have been conducive to disease, fungicide may be used to
help reduce symptoms of NCLB. See the Midwest Vegetable Production Guide for
Commercial Growers for recommendations. Effective fungicides for NCLB include Headline,
Headline AMP and Quilt XCEL. Fungicides
may be less effective if applied after tasselling.
Figure 1: Northern corn leaf blight causes a cigar shaped lesion on the leaves of sweet corn.
I have observed this disease in several pumpkin fields this year. It is not clear to me why this disease seems to be more widespread compared to recent seasons. However, it makes sense to review Plectosporium blight here.
Plectosporium blight is usually not a serious disease. The occurence of this disease is usually sporadic. However, when it occurs, it can cause yield loss if left uncontrolled. Older literature may list this disease as
Microdochium blight. Plectosporium blight can be recognized from the light tan
lesions on stems and leaf petioles. Lesions may also occur on the fruit, although these symptoms are less common. Yield loss is most often caused by lesions on the stem adjacent to the fruit-the pumpkin handle. Yellow
squash and zucchini squash are also affected. Lesions are often individually spindle shaped. When these lesions occur in large numbers they can give a light gray or white appearance to the foilage. This disease
may be managed through a combination of cultural and fungicide treatments. Crop
rotations of 3-4 years and fall-tillage will help keep the crop residue to a
minimum. A regular contact fungicide program will also help to keep
Plectosporium blight in check.
Figure 1: The handle of this pumpkin has lesions of Plectosporium blight which may ruin the marketablity of the fruit.
Figure 2: Plectosporium blight may cause the stems and petioles of pumpkin plants to appear white or light brown when numerous spindle shaped lesions coalesce.
Downy Mildew has been confirmed on jack-o-lantern pumpkins in Daviess and Jasper Counties. This is the first time that this disease has been confirmed on pumpkins in Indiana in the 2015 season. There are unconfiirmed (but reliable) reports of downy mildew on pumpkins in Parke and Washington County. This disease has also been observed on butternut squash in Knox County. Read more about this disease here.
Late blight has been reported on processing tomatoes in Cass County Indiana. This is a late blight update from when this disease was reported on potatoes and tomatoes in LaGrange County Indiana. The latter outbreak and some disease management tips are reported in the Vegetable Crops Hotline here.
Protecting vegetable crops from foliar disease involves
many factors. Crop rotation and fall
tillage will help to lessen disease severity. Choosing a resistant or partially resistant
variety can lower the amount of disease. Purchasing seed that has been tested for seed
borne disease is also an important factor.
Most growers, however, find it is also necessary to apply fungicides to
manage foliar diseases. This article
will discuss when such applications are productive-and one case where they may
Foliar fungicides are most effective when applied before
infection of a plant disease takes place or early in the disease epidemic. That is, it is best to apply fungicides
before one observes disease and at regular intervals. Fungicides are designed to protect healthy
foliage from disease. Applications of
fungicides will not change plant tissue that has been turned brown (necrotic)
from disease into green healthy tissue.
Figure 1 shows a watermelon field with severe symptoms of
anthracnose. Large areas of the field
appear brown due to anthracnose infection.
This situation may have resulted from missed fungicide applications,
lack of crop rotation or weather conditions very conducive to disease. In any case, the common reaction of a grower
upon viewing such a field is to apply a fungicide. Many times the reaction is
to reach for an expensive cure. Again, no fungicide will turn brown plants
into green plants.
When the decision to apply fungicides to a vegetable field
with severe foliar symptoms is made, the grower should make such an application
with the aim of protecting healthy plants.
Is there a large enough area of healthy plants to justify a fungicide
It may be argued that some systemic fungicides have what
may be referred to as ‘kickback action’. That is, the fungicide is designed to
move in the plant and stop fungi inside diseased tissue. Won’t such a fungicide bring plants back from
a diseased situation?
The successful application of many systemic fungicides with
‘kickback action’ will stop or slow fungi inside plant tissue. One should think about such action as
inhibiting fungi at the edge of a plant lesion before it can expand into
healthy tissue. Even if fungi are
stopped inside necrotic tissue, the tissue will remain brown. Thus, fungicides should be applied to protect
green tissue from disease.
When applications of fungicides are made to fields such as
in Figure 1, one should also remember that the disease has almost certainly
spread beyond the dead plants. Since it
takes 7 to 10 days for diseased plants to show symptoms, more of the plants are
diseased than what are observed in Figure 1.
An argument can be made that some crops may be able to
regrow healthy leaves, branches and fruit from diseased plants if kept healthy
with fungicides. The decision to apply
fungicides to allow regrowth of healthy plant tissue will depend of the cost
and efficacy of the fungicide(s), the weather (dry weather will be more likely
to favor healthy growth), market value of the produce and stage of crop.
Applications of a fungicide to non-productive plants may be
made as a way to reduce disease inoculum that may otherwise spread to heathy
plants. If one is realistic about why
such a fungicide is applied, then this type of application can be
justified if the cost is right. Another management tool is to
remove or rogue diseased plants to stop disease spread. But this is a topic for another time.
My intent is not to be pessimistic about vegetable
production or fungicide use. Instead, I
would like to point out the importance of timely use of fungicides, especially
early in the season. In contrast,
fungicides applied late in the season to a crop already very much affected by
foliar disease may not be useful.
Vegetable growers should be realistic about applications of fungicides
to late season crops with significant amounts of disease.
There are large areas of this watermelon field that appear brown from the disease anthracnose. No amount of fungicide will cause these areas to turn green. Growers should assess the likelihood that fungicides will be effective before spending large amounts of money on an application.
Gray mold of tomato is one of the more common diseases of greenhouse-produced tomatoes. Although it is often a minor problem, if left unchecked, gray mold can casue yield loss.
Gray mold, or more properly, Botrytis gray mold, often causes a light gray or brown necrotic lesion on leafs. The lesions on leafs are sometimes wedge shaped on the margin of the leaf. Stem lesions are a similar color and may encircle the stem, causing the death of the upper portion of the stem. Occasionally, gray mold may cause the rot of tomato fruit. Whether on leafs, stems or fruit, the gray fungal sporulation is often easily seen, thus the name. It is a rare symptom, but when fungal spores land on tomato fruit that is wet, the spores may germinate, causing a symptom known as a ghost spot.
Botrytis gray mold can cause disease on many different host plants, enabling the fungus to easily survive and disperse between tomato crops. Host crops include flowers such as geraniums as well as other vegetables such as green beans. The disease is favored by relatively cool temperatures and high humidity which explains why the disease is often observed in greenhouses.
Any cultural practice that lessens humidity such as pruning, will lessen the severity of gray mold. Since gray mold favors older plant tissue, pruning old leafs should reduce susceptible plant tissue. As a general rule, indeterminate tomatoes should be left with no more than eighteen to twenty fully mature leafs after pruning. Determinant (staked) tomatoes are often pruned until the first flower cluster, improving airflow and encouraging larger fruit. Another practice that may reduce airflow is spacing plants too close together.
Practicing crop rotation may reduce the amount of the gray mold fungus that survives in a greenhouse. If crop rotation is not possible, remove as much of the crop as possible far away from production when the season is complete. Clean and sanitize the greenhouse between tomato crops. Use of a ground covering between rows of tomatoes may help to reduce the amount of crop residue that becomes incorporated in the soil.
Several fungicides that may help to manage gray mold are listed in the Midwest Vegetable Production Guide. Products that should be effective against gray mold and are allowed for greenhouse/high tunnel use in Indiana include: Botran, Fontelis, Scala and Switch. Products that contain the active ingredient mancozeb may be less effective than those listed above, but these products may be less expensive and are readily available. Organic producers should look for products that contain formulations of copper.
Figure 1: Leaf lesions of Botrytis gray mold are often a light gray or brown color and the sporulation of the causal fungus can be seen on the leaf margin.
Figure 2: Although less common than leaf or stem lesions, ghost spots may also be caused by gray mold.
This disease has been reported near West Lafayette and in Wanatah
Indiana. Powdery mildew of tomato can be recognized by the white fungal
colonies on both leaf surfaces. Occasionally, stems may also be
infected. Severely affected tomato plants may have leafs that turn
chlorotic and necrotic. Fruit will not be directly affected.
The causal organism has been identified as Pseudoidium neolycopersici, formerly Oidium neolycopersici. This fungus
may survive as resting structures on host material. The spores are easily
wind dispersed to additional tomato plants. Development of this disease
is favored by temperatures below 86F. As with most powdery mildew
diseases, high humidity allow the disease to develop; leaf wetness is not
necessary. Since high humidity favors powdery mildew of tomato,
greenhouse environments often favor the disease.
Reports of powdery mildew on tomato are not common in
Indiana. There is no data on yield loss from this disease on
tomato. Nevertheless, if this disease is present, management options
should be considered. Several systemic fungicides are listed as possible options
in the Midwest Vegetable Production Guide. It might be useful to choose a
product that is also labeled on early blight and Septoria leaf blight. An example would be Fontelis, which is
labeled for field or greenhouse tomatoes.
Figure 1: Powdery mildew of tomato is easily recognized by the white fungal sporulation on both sides of the leaf as well on the stem. (Photo: Natasha Cerruti).
has been observed on watermelon in Knox County.
The following article will discuss the symptoms, biology and management
of downy mildew of cucurbits.
symptoms of downy mildew vary depending on the host.
watermelon, the lesions start out as chlorotic (yellow) areas that become round
and necrotic (brown/black) areas surrounded by a chlorotic halo. Lesions may be limited by veins (Figure 1). Note that leaf lesions of gummy stem blight
may have dark fungal structures (pycnidia) present that are lacking with downy
mildew. Also, whereas gummy stem blight
will affect stems and petioles, downy mildew will not.
lesions also start out chlorotic and are often angular. Eventually, the chlorotic lesions become
necrotic. Lesions may be more common
along a vein.
on muskmelon often have poorly defined margins and are not as angular as
described above for pumpkin.
lesions start out chlorotic and very angular, eventually becoming necrotic.
For all hosts,
the bottom of the lesions becomes covered with a dark ‘fuzz’ under moist
conditions. The ‘’fuzzy’ appearance is
due to the production of spores that are easily wind-borne. Severely affected leafs become crumpled and
brown and may appear scorched. Downy
mildew does not affect stems or fruit directly.
Downy mildew of
cucurbits is favored by temperatures of 59 to 68 F. Relative humidity of 100% for 6 hours is
sufficient to allow infection to take place on a leaf. Once infection takes place, spores are
produced on the underside of the leaf.
The spores are dislodged upon drying and easily become airborne.
organism that causes downy mildew does not overwinter in Indiana; instead it
overwinters on the Gulf coast or in cucurbit greenhouse production somewhere
near the Canadian border. Usually, this
fungus-like organism blows into Indiana in late August or September. This year’s entrance is unusually early.
organism that causes downy mildew of cucurbits does not affect unrelated hosts
such as soybean.
several pathotypes of the fungus-like organism that cause downy mildew. Each pathotype specializes in a different set
of cucurbit hosts. In most years, downy
mildew shows up on cucumbers and cantaloupe first. Since, this situation is a bit complicated
and isn’t entirely understood, I would advise all cucurbit growers in Indiana
to begin the management options described below even if it is only to closely
scout one’s field.
rotation is always a good idea, this management technique will not work for
downy mildew since the causal organism does not overwinter in the soil. There are a few varieties that have resistance,
but only for cucumbers. Most growers
find it necessary to apply fungicides to protect their cucurbit crops. However, since the fungus-like organism that
causes downy mildew is not closely related to the fungi which cause most other
cucurbit diseases, the fungicides that are most effective against downy mildew
are not necessarily effective against most other cucurbit diseases. (A very different fungus causes powdery
mildew of cucurbits.) Products that
can be used to manage downy mildew are listed in the Midwest
Vegetable Production Guide. The most effective products are
likely to include Ranman®, Gavel® and Zampro®. Growers who have been
applying Presidio® for Phytophthora blight may find that this product is also effective
against downy mildew. I have found Previcur Flex® to be effective,
however, more recent data indicate this product to be less effect than the
above products. Products that contain
phosphorus acid (such as Agri-phos, Phostrol, Prophyt, Rappart) may in
situations where the disease is not present, but may threaten in the
future. Contact fungicides such as those
with the active ingredients chlorothalonil or mancozeb may not be as effective
as the products listed above, but do offer some protection. Organic growers should use a formulation of
copper that is certified for organic use.
Additional information can be found through these links.
Figure 1: Downy mildew of watermelon causes a dark brown necrotic lesion often surrounded by a chloritic halo.
Figure 2: The spores from the downy mildew fungus-like organism may cause a 'fuzzy' appearance on the bottom of the leaf such as in the cucumber leaf shown above.
Most growers first notice this disease when large soft areas develop on mature watermelon fruit. These soft, rotten lesions can be several inches across and are often covered with a white mold. The lesions usually form first on the bottom of the fruit, close to where the fruit comes into contact with the soil. Further development of the disease often results in lesions on the top of the fruit as well.
Conditions that favor Phytophthora Fruit Rot include warm, rainy weather such as have occurred over much of Indiana during the past few weeks. Water that stands in pools also favors severe disease symptoms. Overhead irrigation may help the disease to spread. Phytophthora Fruit Rot can spread rapidly when conditions are favorable.
The organism that causes Phytophthora Fruit Rot of watermelon is Phytophthora capsici. This organism is more closely related to algae than to fungi. Therefore, P. capsici is sometimes referred to as a fungus-like organism. The close relationship of P. capsici to algae helps explain why this disease is favored by water; one of the life stages is a spore that is motile is water. Perhaps because of the taxonomic difference between P. capsici and most fungi, the fungicides that are most affective against P. capsici are less affective against most fungal diseases.
To control this disease, choose fields that are well drained, preferably fields that do not have a history of the disease. Watermelon fields with plastic mulch and raised beds may have less disease severity.
Fungicides, if used, are best applied before the disease appears. This is because by the time a few lesions appear, many more lesions are present, but are too small to see. For the most part, growers will want to wait until fruit are present to apply fungicides. Applications to small fruit may include Forum® or one of the products with phosphorous acid as an active ingredient (e.g., Agri-Phos®, Phostrol®, Kphite®). However, during conducive conditions (such as rainy weather on fields with a history of the disease) Presidio® can be alternated with Revus®. A newer product, Zampro®, has also proven effective.
Be careful to alternate fungicide modes of action, or FRAC codes. For example, Revus®and Forum® both have 40 FRAC codes and should not be alternated. Zampro® has two FRAC codes, 40 and 45, and therefore should not be alternated with Revus® or Forum®.
Phytophthora Fruit Rot of watermelon can be a serious disease. Prepare for this disease before it appears in a field near you.
Figure 1: Phytophthora fruit rot of watermelon causes large, soft lesions on the surface of watermelon fruit. Under moist condiitions, the lesions may be covered with a white mold.