Category: Vegetable

Heat Stress on Plants

Emily Zobel on

While part of Maryland got some rain this week, the past two weeks have been hot and dry so that you might see symptoms of heat stress on plants. 

Heat stress occurs when plants are exposed to enormous amounts of sunlight and heat for an extended period of time. The ideal range for most crops is between 68 and 86 degrees Fahrenheit (20 and 30°C). Temperatures outside this range, whether in the air or the soil, during the day or the night, are harmful to plants. The term “heat stress” often refers to a period in which plants are subjected to high temperatures long enough to alter their ability to function or grow normally.  

Heat stress disrupts various plant processes, such as inhibiting the growth and development of plant tissue, reducing the photosynthesis rate and nutrient uptake, reducing pollen production, causing flower and fruit drop, and causing sunscald on fruit.  Shallowly rooted and young plants will be the most impacted by heat and drought. 

Common visual signs of heat injury in plants include the following: leaf rolling and cupping, leaf drop, scalding and scorching of leaves and stems, rapid leaf death, reduction in growth, and lower yields. Wilting is the primary sign of water loss, which can lead to heat damage.

Drought and heat stress on green bean plants.
Photo by Howard F. Schwartz, Colorado State University, Bugwood.org

High temperatures can also cause increased sunburn and other developmental disorders in fruits and vegetables. High nighttime temperatures decrease pollen production, reducing fruit sets and smaller fruit. Flower abortion can happen at temperatures ranging from 75° F to 95° F, depending on the crop.

Some vining vegetables in the cucurbit family (pumpkins, squash, melons, cucumbers, and the like) will develop more male flowers than female flowers in high temperatures (over 90° F during the day and 70° F at night).

REDUCING PLANT HEAT STRESS

  • A great way to reduce heat stress on plants is by meeting their water need. Plants keep cool through the evaporation of water from their leaves. If a plant lacks water, it will close the pores in its leaves (stomates), preventing evaporation to avoid wilting. When stomates close, the plant can no longer keep itself cool and leaves heat up, causing stress. Morning watering is often prescribed in times of high heat.

  • Using overhead watering, sprinkling, and misting can provide humidity, which we typically avoid to decrease plant disease; however, in prolonged high temperatures, the benefit of a humid environment probably outweighs the risk of plant disease.
  • By adding compost to your soil, you can increase its organic matter and, in turn, its water-holding capacity.

  • Mulches can also help reduce heat stress by increasing the reflection and dissipation of radiative heat. Reflective mulches like straw and wood chips can reduce surface radiation and conserve moisture.   Black plastic mulch is excellent for heating the soil in the spring to allow for early plant growth. However, its surface temperature can exceed 130°F on a hot sunny day, resulting in injury or desiccation of most plant parts (root, stem, leaves, and fruits) in direct contact with the mulch.  White or metalized plastic can be used for summer crops that are more at risk of heat damage. However, these plastic mules tend to be more costly.  An alternative to white and metallic mulches could be to make a foliar application of kaolin clay. The white coating of kaolin clay would serve as a reflective layer on the black mulch and plant canopy to minimize temperature changes.  A study conducted at the Eastern Shore Agricultural Research and Extension Center in Painter, Virginia, found that 20 to 40 lb/A of kaolin clay could maintain 10⁰F cooler soil temperatures compared to black plastic.

  • Shade cloth has been shown to significantly benefit heat-sensitive crops if applied correctly by reducing the soil and air temperatures around them. Shade cloth can be placed over crops, using stakes or hoops as support; it can be particularly beneficial to sun-sensitive crops such as peppers and tomatoes. Research at UDel by Dr. Emmalea Ernest found that a 30% shade cloth provides adequate cooling without blocking too much light for most vegetables.

  • Avoid applying fertilizers and pesticides when temperatures are consistently above 80°F, as they can burn crops. Check the product’s label for specifics before applying. 

Spinach Crown Mites in Spinach

Emily Zobel on

Spinach Crown Mites in Spinach
by Jerry Brust, UME

Spinach crown mites Rhizoglyphus sp. feed within the folds of new leaves in the crown of spinach plants. This feeding causes the new leaves to become deformed as they grow (figs.1 and 2). Crown mite adults are extremely small bulbous nearly transparent mites that also may have a yellow-beige body color with reddish-brown legs (fig 3). A good characteristic to look for to identify these mites is the sparse long hairs mostly found on the back end of the mite (fig. 3). Crown mite eggs are spherical and clear and laid on the creased leaf surfaces in the crown area. Some reports state that crown mites can act as vectors for plant pathogens such as Pythium and Rhizoctonia, but this is not definitive.

Feeding by spinach crown mites can cause misshapen and ragged leaves with necrotic margins as they expand and crown leaves are distorted and wrinkled in appearance.
Figs. 1 & 2.) Crown leaves fed on by spinach crown mites are misshapen and ragged with necrotic margins as they expand and in the field the crown leaves are distorted and wrinkled in appearance. Photos by G. Brust, UMD.

The spinach crown mite is most damaging in soils high in organic matter and under cool moist conditions (weather conditions we have had this past week). Because these mites can consume organic matter they can survive in soils after the crop has been removed. This is one reason they are difficult to control as they can survive for fairly long periods of time with no crop being present. The other reason they are difficult to ‘control’ is we do not realize they are causing the problem until it is too late.

Spinach crown mite adult with sparse long hairs over its body.
Fig 3.) Spinach crown mite adult with sparse long hairs over its body. Photo by G. Brust, UMD.

Most control recommendations include sanitation and crop rotations as being important as are fallow periods. Pyrethroids are a possible chemical control as is Neem; any chemical control has to get down into the crown of the plant to have any chance of working. There has been little research conducted on the most efficacious material for these mites. Mostly what is needed are warm sunny days where spinach can grow well and the environment is not so conducive to the mites.

Stinkbug Damage Found in Maryland Tomato Fields

Emily Zobel on

Stinkbug Damage Found in Maryland Tomato Fields

Jerry Brust, UME

There have been reports of stinkbug damage in tomatoes in Maryland for the last couple of weeks. Stinkbug feeding damage is called cloudy spot in tomato fruit (fig. 1). It occurs when the adult or immature stinkbug puts its needle-like mouth part into the fruit and removes material from a large number of cells. On green fruit the damage appears as whitish areas with a black dot in the center and indistinct borders (fig. 2) on ripe fruit the spots are golden yellow (fig. 1). Individual spots may be 1/16 -1/2 inch in diameter; or the spots may merge and encompass a large area of the fruit surface (fig. 2). Peeling back the skin shows these areas as white shiny, spongy masses of tissue (fig. 3). This damage is usually most common from mid-July until the end of the season.

Stinkbug feeding injury to tomato fruit.
Fig.1) Stinkbug injury to tomato fruit. Photo by G. Brust UMD.

The Green and Brown as well as the Brown Marmorated stinkbugs are often difficult to see and usually go unnoticed as they spend much of the day deep inside tomato plants, any disturbance and the stinkbugs will drop to the ground and move under the plastic, which results in monitoring difficulties. Only a few are necessary to cause the appearance of cloudy spot on many tomato fruit. Although stinkbug damage has been observed in slightly greater than usual amounts in tomato fields this year, observations of stinkbugs have been much less numerous.

Continue reading Stinkbug Damage Found in Maryland Tomato Fields

Tomato and Blossom End Rot

Emily Zobel on

By Jerry Brust, IPM Vegetable Specialist, University of Maryland

This is just a reminder with the usual summer weather we have had lately with stretches of very high temperatures and the occasional very heavy down pour we have had over the last few weeks blossom end rot can become a real problem in tomatoes. Sunny days will suck water through a plant quickly and the downpours will disrupt calcium movement through the plant. The key is to try and maintain consistent soil moisture while the fruit is developing. Easier said than done I know, but soil moisture levels need to be monitored as best as they can. When you see something like figure 1, with all the tomatoes on a cluster with blossom end rot you know the soil moisture fluctuated greatly over a fairly long period of time. Applying some foliar calcium sprays may help, but the applications can’t overcome poor soil moisture management.

Cluster of green tomatoes with blossom end rot ( browning at the base).
Fig. 1 All the tomatoes on this one cluster have blossom end rot—indicating poor soil moisture management. Photo By J. Brust. 

Blossom End Rot Common so Far this Season.

Emily Zobel on

Blossom End Rot Common so Far this Season.

By Jerry Brust, UME

This summer has been unusual as it has been about normal for temperatures if not a little cooler, but we have had greatly varying amounts of rainfall over the last month. Some areas have remained dry with storms just missing farms while others have been hit with some heavy rains. This can make watering vegetables challenging to avoid problems such as blossom end rot, which is caused by a calcium deficit in the developing fruit.

Figure 1. Fig 1. Several different vegetables with blossom end rot.

Calcium (Ca) moves to the plant via mass flow, i.e., where dissolved minerals like calcium move to the root in soil water that is flowing towards the roots. As it passes through the plant Ca is deposited in tiny amounts into the fruit. If anything slows or interrupts this stream the tiny amount of Ca needed at that moment is not deposited and the area furthest from the top of the fruit suffers—resulting in blossom end rot (BER). I have seen more BER this year on a large number of different vegetables than I have in the past several years (fig 1).

Figure two shows how precise and constant the Ca flow in a plant has to be to supply just the right amount of Ca at the right time. The large fruit on this particular plant developed before there was a Ca interruption, but the fruit a little younger suffered a Ca interruption, with the smallest (youngest) fruit suffering the greatest Ca interruption. At the time it was taken tissue analysis from this same plant showed that calcium was in the moderate range when the blossom end rot took place, demonstrating the importance of irrigation and water supply to reduce blossom end rot. Not much you can do about no rains or heavy rains, except try to maintain as even a water supply to your vegetables as is possible and remove any fruit from the plant you find that has blossom end rot.

Fig. 2 Older larger fruit received enough Ca, but younger (smaller) fruit did not so they are showing signs of blossom end rot.

 

 

 

 

 

Tomato Pith Necrosis in Maryland

Emily Zobel on

Tomato Pith Necrosis in Maryland

By Jerry Brust, UME

In the last week a few tomato fields in Maryland were found with the same disease called tomato pith necrosis. Just about all the problem tomatoes were from early planted fields. Tomato pith necrosis is caused by the soilborne bacterium Pseudomonas corrugata. Although in the past this disease occurred sporadically in Maryland, over the last few years it is appearing more frequently. Tomato pith necrosis usually is found in early planted tomatoes when night tempera­tures are cool, but the humidity is high, and often plants are growing too rapidly because of excessive nitrogen application. We have had a spring/early summer with some cool nights and high humidity.

Fig. 1. Beginning of pith necrosis- leaves anywhere on plant can turn yellow. Photo by G.Brust, UMD

Continue reading Tomato Pith Necrosis in Maryland

Tomato Spotted Wilt Virus in Tomatoes

Emily Zobel on

Tomato Spotted Wilt Virus in Tomatoes

By Jerry Brust, UME

A few high tunnels and even a couple of tomato fields have been found with tomato spotted wilt virus (TSWV) in Maryland. The high tunnel finds were not too surprising

but the fields were, as we usually do not see field infections until much later into the season. TSWV has also been found in greenhouse and field production of cut flowers. So it appears this virus is more common this year than it usually is, most probably due to greater thrips populations being present in our greenhouse production areas.

Fig. 1 Tomato leaves with TSWV symptoms

Continue reading Tomato Spotted Wilt Virus in Tomatoes

Allium leaf miner

Emily Zobel on
Adult allium leaf miners.

Allium leaf miner Phytomyza gymnostoma is a pest on chives, scallions, garlic, onions, and leeks. Overwinter ALM across the Mid-Atlantic will be emerging soon. Start scouting for ovipositor markings made by female ALM over the next few weeks on food and ornamental Allium crops. These markers will be neat rows of white spots descending from the upper tips of allium leaves. These marks will be typically be seen on the tallest leaves first. ALM Larvae mine leaves and moves towards and into bulbs and leaf sheathes. The leaf punctures and mines serve as entry routes for bacterial and fungal pathogens.

Spring crops are usually not as hard-hit as fall crops, especially when looking at leeks, but this pest has steadily increased its geographical range each year and its damage potential. If you had some infestation last year, you will especially want to look for this pest’s signs.

Yellow sticky cards or yellow plastic bowls containing soapy water can be used for monitoring but are not affected control independently. 

 

Onion leaf blades showing round white dots made by female Allium leaf miners. Photo by Jerry Brust

 

Cultural Control: Covering plants in February, prior to the emergence of adults, and keeping plants covered during spring emergence, can be used to exclude the pest. Avoiding the adult oviposition period by delaying planting (after mid-May we think) has also been suggested to reduce infestation rates. Covering fall plantings during the 2nd generation flight can be effective. Growing leeks as far as possible from chives has been suggested. Organic

 

Chemical Control: Azadirachtin (Aza-Direct or other formulations) or spinosad (Entrust or other formulations) follow label instructions for leaf miner.

Synthetic Chemical Control: Systemic and contact insecticides can be effective. EPA registrations vary, however, among Allium crops. Check labels to ensure the crop is listed, and for rates and days-to-harvest intervals. Options that may be effective include cyromazine (Triguard), dinotefuran (Scorpion), spinetoram (Radiant), lambda-cyhalothrin (Warrior II or other formulations), and abamectin (Agri-Mek or other formulations).

Plant detective: Why are the leaves of my vegetables turning yellow?

Emily Zobel on

By Haley Sater
Agriculture Agent, Wicomico County
University of Maryland Extension
HSater@umd.edu

Everyone with a garden knows how exciting it is when you first get going in the spring. Whether you started your vegetables from seed or as transplants the first week or two all your little plants look promising. Until, inevitably, something goes wrong. This year for me, it was chlorosis or leaf yellowing in my garden. I first noticed it because it was very prominent on the leaves of my zucchini. Once discovered, my plant sleuth diagnostician training kicked in. Immediately, I started looking around and trying to gather several key informative clues from the garden to help determine what the problem could be and how to fix it.

Zucchini plant with yellowing leaves. Photos by H. Sater
Zucchini plant with yellowing leaves. Photos by H. Sater

 

First you must define the problem: I saw leaf yellowing in my zucchini, but wait, it appeared to only be affecting the new leaves, not the old leaves. The leaf yellowing seemed to be affecting all four zucchini I have planted equally. There didn’t appear to be any insect damage or lesions or spots on the leaf, just a lack of green pigment in the new leaves and flower buds of the zucchini plants. 

Next I looked around the rest of the garden trying to see if I could find any similar issues in my other vegetables and berries. Sure enough similar but less pronounced leaf chlorosis appeared on a couple of my raspberry plants and tomatoes. 

Next step: Now it’s time to think like a doctor, which means asking the right questions. First, we break down all the things that can kill plants into two categories: abiotic and biotic. Abiotic means environmental conditions, and biotic being any kind of living organism that can act as a pathogen. In general if you have a biotic problem like a fungal disease or insect herbivory there will be specific symptoms like insect holes or trails on the leaf. A fungus might cause spots which will eventually make powdery spores on the surface of the leaf. These signs of pathogens are often not symmetrical. However, the leaf yellowing present on all of these plants appeared to be mostly symmetrical and lacked any tell-tale signs of insect or disease, pointing me in the abiotic direction. Additionally, the same symptoms were visible in multiple plant species across the garden. Generally, different species of plants that are not closely related will not all suffer from the same types of biotic pests, but if there is an abiotic problem like drought or nutrient deficiency, it is more likely that it will be found across the garden in multiple species. 

Form a hypothesis: I began to suspect that I had a nutrient deficiency. Nutrient deficiency shows themselves in different ways depending on which nutrient is missing. However, chlorosis or leaf yellowing is a very common symptom of several nutrient deficiencies. Additionally, I had neglected to get a soil sample taken of my garden in the spring and so I knew there was a possibility that I could be lacking some unknown nutrients. 

Nutrient deficiency key adapted from image from Heidi Wollaeger, Michigan State University Extension

Next, I used a deficiency key to decide which nutrient might be missing or unavailable to my plants in the soil.  

Q1.) Are older or younger or all leaves affected? A1: Youngest 

Q2.) What’s wrong with them? A2: Chlorosis/ yellowing. 

Q3.) Is the chlorosis everywhere except the leaf veins? Or does it have tan specking?  A3:  No tan specking, no chlorosis on the leaf veins. 

So the nutrient that is missing is iron (Fe). 

Deficiency chart from University of Arizona Cooperative Extension

 

Healthy zucchini plant with green leaves.
Healthy zucchini plant. Photos by H. Sater UME.

One month later

After the application of a micronutrient fertilizer which included iron my zucchini are looking so much better. I can’t be sure that the missing nutrient was iron because the fertilizer I used was a blend of different micronutrients, but it sure made a difference. The yellowed leaves have regained their normal green color; which just goes to show that nutrient availability in your soil can make a big difference in the health of your garden.