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.
Cabbage is the crop most often affected by black rot, however, other crucifers such as broccoli, cauliflower, mustard, Kohlrabi or Brussel sprouts may be affected. The first symptom one is likely to notice is a ‘V’ shaped lesion on the margin of the leaf (Figure 1). However, the symptom on Brussel sprouts observed recently are irregular, jagged lesions on leafs (Figure 2). The plants represented in Figures 1 and 2 are different varieties of Brussel sprouts. The differences may be due to differences in susceptibility of the two cultivars or the cultivar in Figure 2 may have been infected at an earlier age than the one in Figure 1. Figure 3 shows two severely affected plants next to a relatively healthy plant.
Black rot is most severe in wet, warm weather. The emergence of this disease during a rather cold spring may mean that the disease started in a greenhouse situation.
The bacterium that causes black rot, Xanthomonas campestris, survives in crop residue. Thus, crop rotations that avoid crucifers should lessen the severity of the disease. Sanitation in the greenhouse should help to lessen the amount of the bacterium that can cause more disease. The causal bacterium may also be transmitted through seeds, therefore, every effort should be made to plant seed that has been tested and found free of the bacterium. Inspect transplants for symptoms before planting. Avoid practices which add to free water to plant surfaces. Products which contain copper as an active ingredient may help to lessen the spread of the disease. However, copper products may also cause lesions on leafs under some circumstances. Although there may be varietal differences in susceptibility, compete levels of resistance are not available in commercial cultivars. More information about general pest management can be found in the Midwest Vegetable Production Guide for Commercial Growers, 2015. Midwest Vegetable Production Guide for Commercial Growers, 2015.
Figure 1: Typical symptoms of black rot include 'V' shaped lesions on the margins of leaf.
Figure 2: The angular lesions on this Brussel sprouts leaf are not typical of black rot.
Figure 3: The two Brussel sprouts plants on the left are severely affected by black rot.
This disease has been reported in two different greenhouse situations. Although the disease is not usually economically important, a brief review of the disease is offered here to help tomato growers differentiate pith necrosis from more important problems.
Tomato pith necrosis causes dark brown streaks on tomato stems and leaf petioles (Figure 1). Often stems may appear twisted and distorted. When cut open, the stem may appear discolored and chambered (Figure 2). Eventually, the affected plant may become stunted and wilt. Tomato pith necrosis is usually found in greenhouses or high tunnels.
Because the plant has a discoloration in the stem, it is sometimes confused with bacterial canker, a much more serious disease. A comparison of the two diseases can be found here.
It is not clear how pith necrosis spreads or enters the tomato plant, but it is probably best to remove affected plants and avoid using pruning equipment on diseased plants. When removing diseased plants, it is always best to leave as little of the plant behind as possible. A landscape cloth covering can help to keep crop residue out of the soil.
Perhaps since pith necrosis is not economically important and does not appear to spread quickly, not much is listed for the management guidelines. To manage tomato pith necrosis, avoid low night temperatures and excessive nitrogen levels; reduce high humidity in the greenhouse or high tunnel. General pest management guidelines can be found in the Midwest Vegetable Production Guide for Commercial Growers 2015.
Figure 1: Pith necrosis of tomato may result in dark, necrotic streaks on stems.
Figure 2: Pith necrosis of tomato may cause internal discoloration and a chambered internal stem.
Fusarium wilt of watermelon is often observed on about
Memorial Day in Indiana. This is because
at this date, watermelon vines are usually 6 to 10-inches long, the stage where
Fusarium wilt often shows up. This year,
most growers are behind in their planning due to the cold, wet weather. The Fusarium wilt I observed this week were
on vines that had been under low row covers.
In addition, last week I also observed Fusarium wilt of watermelon still
in transplant trays.
Symptoms of Fusarium wilt of watermelon include wilting and stunting. Often
one leaf or vine may be wilted leaving the rest of the plant apparently healthy
(Figure 1). Seedlings that are affected are often not big enough to show such
one-sided symptoms (Figure 2). While the
roots may be white and healthy, the interior of the stem may have a brown discoloration.
Relatively cool weather seems to favor the fungus over the watermelon seedling.
Thus, cool weather in May and early June sometimes leads to a higher incidence
of Fusarium wilt. In addition, this disease is often found in well-drained
areas. Regardless of how many watermelon plants appear to be dying at the
moment, remember that this disease has not been shown to spread from plant to
plant in the field. Typically, no more than 10 percent of plants will be killed
due to Fusarium wilt of watermelon. In addition, once the weather turns warmer,
the watermelon plant will begin to outgrow the fungus. It is possible to spread
the fungus that causes Fusarium wilt of watermelon through soil that remains
attached to cultivation equipment between fields. In order to minimize the
spread of the fungus between fields, clean off soil between fields with
high-pressure water. It may not be practical to disinfest the equipment between
fields; however, one might spray a solution of a quaternary ammonia solution
(Greenshield or Physan 20) or 10 percent bleach on the tools. Growers will want
to make every effort to keep infected seedlings out of commercial fields.
Closely inspect transplants before planting them. Fusarium wilt can be
transmitted on seed. Previously used trays may harbor the fungus that causes
Fusarium wilt. Although transplant trays can be disinfested, it can be very
difficult to clean and disinfest trays sufficiently to eliminate the
possibility of Fusarium wilt.
Use long crop rotations of at least 5 to 6 years between
watermelon crops since the fungus that causes Fusarium wilt survives for
several years without a host planted in the field. The fungus that causes
Fusarium wilt of watermelon is very specific to watermelon and will affect no
other crop plants. Likewise, Fusarium of other crop plants like tomato and
cabbage will not affect watermelon. While no variety is completely resistant to
Fusarium wilt of watermelon, there are differences in susceptibility. The Midwest Vegetable Production
Guide for Commercial Growers 2015 has more information about Fusarium wilt
of watermelon including a soil applied fungicide. This extension
bulletin also has information about Fusarium wilt of watermelon. Feel free to call me with
comments or questions.
Figure 1: Fusarium wilt of watermelon often causes one vine to wilt while the remainder of the plant appears healthy.
Figure 2: Fusarium wilt may also occur on watermelon seedlings in transplant trays.
Most watermelon growers are in the process of placing
transplants in the field. I have
received several commercial samples of transplants still in trays prior to
out-planting. The two diseases I have
observed so far are gummy stem blight and bacterial fruit blotch. Below, I discuss these two diseases as well
as management options.
Gummy stem blight on transplant seedlings may be recognized
by the watersoaked area of the stem (botanical term: hypocotyl) as shown in Figure 1. The watersoaked area may eventually turn
brown and woody. A closer look at the
woody area may reveal the small, dark fungal structures of the gummy stem
blight fungus (Figure 2). The true
leaves of watermelon transplants may also be affected.
The fungus that causes gummy stem blight (Didymella bryoniae) may survive in crop
debris, thus overwintering in the field from year to year. This fungus may also survive in seed. It is also possible for the fungus to survive for short periods in greenhouse production facilities.
Crop rotations with non-cucurbit crops for 3 years will help
to lessen disease severity. Preventive
fungicide applications may be scheduled with MELCAST, a weather-based disease
forecasting system. Contact fungicides
such as chlorothalonil (e.g., Bravo, Echo, Equus, Initiate) or mancozeb (e.g.,
Dithane, Manzate, Penncozeb) should be alternated with systemic products such
as Luna Experience, Switch, Inspire Super or tebuconazole (e.g., Monsoon). Not all of these fungicides are labeled for
other fungal diseases such as anthracnose.
Remember to alternate modes of action by using the FRAC codes of the
fungicides. See the Midwest Vegetable Production Guide for more
The symptoms of bacterial fruit blotch (BFB) can be
difficult to recognize on foliage. Leaf
lesions may be angular and appear to run along the vein (Figure 3). The lesions may appear watersoaked,
especially when viewed on the underside of the leaf. Leaf symptoms of BFB are easily confused with
angular leaf spot, a disease that is not often economically important. A laboratory analysis may be required to
distinguish these two diseases. The relatively
large, oily lesions on fruit are easier to recognize (Figure 4).
In contrast with gummy stem blight described above, the
bacteria that causes bacterial fruit blotch (Acidovorax avenae subsp. citrulli) does not readily survive in crop
residue. The bacterium is known to survive
in seed. It is possible that the
bacterium may survive in greenhouse production facilities for short periods.
Once BFB is detected in the field, applications of a copper
product tank mixed with a mancozeb
product may help to lessen disease severity.
Whether BFB is detected in a watermelon field or not, updated
recommendations are to apply copper 2 weeks before first female bloom, at first female bloom
and 2 weeks after first female bloom. Additionally, application of the product
Actigard at 2 of the 3 copper application times listed above is recommended.
More information about these new recommendations may be found here or in the
Midwest Vegetable Production Guide.
Be sure to inspect seedlings for signs or symptoms of
disease. Avoid planting transplants that
may be diseased.
Figure 1: This
watermelon transplant has a water soaked area just under the seed leaves, a
typical symptoms of gummy stem blight.
Figure 2: A more
advanced symptom of gummy stem blight is the light brown woody appearing area
of the stem near the seed leaves. The
dark structures of the fungus that causes gummy stem blight may be observed
with a 10X hand lens.
Figure 3: Lesions of
bacterial fruit blotch of watermelon on transplants may include angular lesions
that may appear water soaked.
Figure 4: Mature
watermelon fruit may have large, dark, irregular lesions due to bacterial fruit
Symptoms of this disease include tomato plants with lower
leaves that become yellow (chlorotic) and die; plants that begin to wilt; a
lesion on the lower stem at ground level (Figure 1 and 2).
If tomato plants are removed from the soil and carefully split open from
the ground level, a discoloration of the vascular tissue can be observed (Figure 3). It is important to note that this
discoloration does not extend up the stem more than 6 to 8 inches. If the discoloration extends up into the
plant canopy, the disease maybe Fusarium wilt of tomato. Although growers may observe multiple plants
begin to die of this disease over a period of days or even weeks, the fungus
does not splash from plant to plant.
Therefore, there should be no plant-to-plant spread in the high tunnel.
Temperatures from 68 to 72 degrees F favor Fusarium crown
rot and may explain why I observed this disease last week when the weather was relatively
cool. I often observe Fusarium crown
rot in high tunnel or greenhouse situations where the tomato plants are grown
in the ground. This is because the
causal fungus, Fusarium oxysporum f.sp.
radicis-lycopersici, survives very
well in the soil in the absence of the host.
Crop rotations that do not include tomatoes or other
solanaceous crops will help to lower the amount of fungal spores in the soil.
However, since the causal fungus survives for years without a host, crop
rotation is not a complete solution. I
also realize that many growers who produce tomatoes in high tunnels do not feel
it is economically practical to rotate to another crop. To such growers, I would point to this
article about how to minimize diseases in high tunnels.
Growers who plant tomatoes in bags or pots in a high tunnel instead
of in the soil should avoid Fusarium crown rot since the fungus survives in the
Check with your seed representative or seed catalog for tomato
varieties with resistance to Fusarium crown rot. Most tomato varieties with resistance to
Fusarium crown rot are indeterminate. (In
contrast, there are many varieties with host resistance to Fusarium wilt.) It is possible to graft your favorite tomato variety
as a scion onto a rootstock variety with resistance. This table
will help one select tomato rootstocks with resistance to Fusarium crown rot
and other diseases. Some tomato seed
companies will sell grafted tomatoes.
There are no fungicides to control Fusarium crown rot. Most fungicides are for foliar use; I know of
no fungicides that may be sprayed on the top of the soil. Read the label carefully and contact me if
you have questions.
Figure1: The tomato plants shown here are stunted, wilted and the lower leafs are dying due to Fusarium crown and root rot.
Figure 2: The lesion at the base of the stem is typical of Fusarium crown and root rot of tomato.
Figure 3: Vascular tissues are discolored in this tomato stem as a result of Fusarium crown and root rot. Note that discoloration only goes a few inches up the stem. Tomatoes with Fusarium wilt have a similar discoloration that goes up into the canopy of the plant.
Last year at the Southwest Purdue Agricultural Center
(SWPAC) we conducted a tomato high tunnel trial described here. In this article, I would like to talk about
the trial we will conduct in 2015, a repeat of the 2014 trial. In particular, I would like to talk about
what we have done for fertility.
Before deciding on a fertility scheme, it is critical to
conduct a soil test each year. Our soil
test from November 2014 showed that our high tunnels were low in sulfur, boron
and moderately low in zinc. In fact, plant
tissue tests conducted during the 2014 season were low for both sulfur and
boron. As a result of these tissue tests, we added a
10% liquid boron product and ammonium thiosulfate (7%) to the fertigation
during the 2014 season. However, the next set of tissue tests carried out
during the 2014 season also came back low in these two elements. It wasn’t until the end of the 2014 season
that we observed levels of boron and sulfur close to normal. It may be that when tomato plants are growing
very quickly, it is difficult to add sufficient nutrients to keep up with
demand. (I should add that our yields of tomatoes were over 140,000 lbs on a
per acre basis for the 2014 season. The
low boron and sulfur tissue tests didn’t seem to hurt our yields too much. However, if we hadn’t monitored by tissue
tests and added boron and sulfur, the yield may have been affected.) Since we
had trouble keeping up with boron and sulfur levels in 2014, this year we
decided to add 2.5 lbs per acre of zinc sulfur (10/7%) and 1.5 lbs. per acre
boron (14.3%) pre-plant broadcast (see update below). We
also added 200 lb per acre pelletized lime.
During the season we will add nutrients at every irrigation
(fertigation). We transplanted on April
2, 2015, adding a cup of 20-20-20 liquid starter fertilizer per plant. We started fertigating potassium nitrate (KNO3)
on April 6. We mix 2 oz. of KNO3
per gallon which is then applied at a ratio of 1:100 at each fertigation. Each high tunnel has 5 rows 80 feet long that has drip tape and black plastic mulch. We started out fertigating 20 gallons twice a
day per high tunnel. Five days later, we started
fertigating 20 gallons 3 times a day. On April 21, we started giving the tomatoes 30 gal 3 times per day.
We try to avoid fertigating with pre-mixed products such as
a 20-20-20 through the drip. Such
products almost invariably add elements that are not needed. In our case, for example, phosphate is not
needed and would be added in most general mixes. Adding elements that are not
needed may lead to a buildup of salts in the soil. This is a particular problem where tomatoes
are grown year after year in a greenhouse or high tunnel.
Please return to this blog to hear about other developments
in our high tunnel projects or other vegetable issues that I encounter during
the season. And feel free to contact me
with questions or comments.
Update June 5-We just recived our first foliar nutrient tests back. In many of the samples, the boron was listed as 'excessive'. We don't see any advserve reaction of the tomato plants yet, but it looks as if we over did the boron.