Last year, I accidentally introduced bacterial leaf spot into the pepper beds in my own community garden plot and the Derwood Demo Garden, probably through infected seed. BLS is a common disease of peppers that spreads quickly in warm, humid weather (a.k.a. our summers) and can be devastating to an entire crop, especially in small gardens. Lesions on leaves expand until the foliage drops; lesions on fruits make them unattractive though still edible. Cooler, drier weather (the kind we mostly don’t get) can slow or even stop the disease, but otherwise, the only solution once it really gets going is to pull the plants. Trash them, don’t compost them, because while the disease doesn’t linger long in soil, it can last quite a while in plant debris. This also means that garden cleanup is important. Read more about BLS and how to prevent and manage it here.
I mentioned in my post last month that many of the pepper plants I grew from seed this year were infected by bacterial leaf spot. I did my best to remove diseased material from the garden, but undoubtedly some of it still lingers in the soil and might survive to infect plants next year, so I will plan to grow varieties that are resistant to this disease.
Pepper leaves infected by bacterial leaf spot. Photo from HGIC.
And no, I don’t just have a list of these varieties in my head. I’ll have to do some research. Maybe you’ve faced this problem too, when all or some of your tomatoes or squash or some other vegetable succumbed to a disease you managed to get identified—and your friendly Master Gardener or Extension specialist advised you to plant resistant varieties. How do you go about finding them?
First, check out Cornell Cooperative Extension’s Disease Resistant Vegetable Varieties page. Here you can find a list for each of many common vegetables of available varieties and the diseases to which they’re resistant. For example, if you have a problem with fusarium wilt in tomatoes, click on Tomato and then skim down the list to note all the varieties with fusarium wilt resistance. (Lucky you; there are lots.) You can also download the information in spreadsheet form for easy reference.
If you have favorite seed catalogs, they can be another good source. Most seed catalogs will add codes to each variety listing that represent disease resistance; for example, fusarium wilt is coded as “F” and if you see “F1-2” that means it’s resistant to both races 1 and 2 of fusarium. (The more resistance, the better, since you probably don’t know which race infected your plants.) Somewhere in the catalog section for each vegetable there will be a list of codes and what they mean.
If you don’t grow your own plants from seed, use these resources anyway. Make a list of resistant varieties, find a garden center or other plant source with a wide selection, and pick out the plants that meet your criteria. Make sure the plants you select look healthy!
Here are some other things to keep in mind:
Hybrid varieties are more likely than heirloom or other open-pollinated varieties to have demonstrated disease resistance. If you have anecdotal evidence that an heirloom is pretty resistant, try it out, but you might want to grow it separately from your bed of resistant types. Same goes with that variety you just have to grow because you can’t do without it, but you suspect it may come down with the bug—grow it away from the others.
“Disease resistant” does not mean “can’t possibly get the disease.” If the pathogen is present, it will probably still infect plants, but the resistant plants will stay healthy much longer—maybe until the very end of the season, if you’re lucky.
Know the source of your pathogen: is it present in the soil, or will it reappear blown in on the wind, or is it more likely to come from infected seed or plant material? Knowing a bit about how transmission works will help you strategize to keep plants healthy. The HGIC website is a good source for information about plant diseases and how they spread (hint: look at the entry for the vegetable and it will list relevant diseases).
There may be other methods available to keep disease away, such as treating seeds with bleach or hot water, using fungicides, and most importantly keeping soil around plants covered with mulch to prevent splashing onto leaves. Also try to avoid overhead watering; water at the base of the plant. You can also remove and destroy infected plant parts when you spot them. Using these methods in combination with planting resistant varieties will increase your odds of success.
Do your research and have a bountiful, disease-free growing season next year!
By Erica Smith, Montgomery County Master Gardener. Read more posts by Erica.
Sometimes individual stems wilt on otherwise healthy perennials. Photo: C. Carignan, University of Maryland Extension
Q: Why would my perennial Lobelia be only partially wilting? I keep an eye on watering so it doesn’t get too dry.
A: This can often be the work of an infection or, less commonly, stem-boring insects. Since stem-boring insects in perennials are rarely a cause for concern, I’ll focus on the pathogens.
Sometimes it’s hard to diagnose the cause of wilting with certainty since distinctive symptoms identifying the culprit aren’t always visible; there’s a lot of symptom overlap. During summer, heat-loving fungi like Southern Blight can cause wilting because the pathogen kills the stem tissue close to the soil line, cutting the top growth off from its water supply and causing it to collapse. A wide variety of fungi and fungus-like “water molds” can cause above- or below-ground infections throughout the year, with some prospering in cool conditions while others need heat. Wilting when the soil isn’t dry can be a telltale sign of infection or physical stem or root damage.
To reduce vulnerability to diseases like Southern Blight, plant crowns need to get good airflow. (The crown is where stems emerge from the root system.) Avoid over-mulching, which covers up or crowds the crown and stem bases; nudge it a few inches away so it’s not piled against the stems. High humidity and dew are sometimes enough moisture for fungi to survive and begin infections, but over-watering plants by irrigating too often can make fungal outbreaks more likely. (This includes lawn grasses, if sprinklers are run frequently.)
Mycelium, which is the “body” of a fungus, is sometimes visible as white webbing or threads covering the infected part of the plant, but it’s not always prominent. (It is usually noticeable with Southern Blight.) Any wilting or collapsed stems need to be removed, since they will not recover. Prune them off and dispose of the debris; don’t compost it.
One or two wilted stems doesn’t necessarily doom the entire plant patch or garden bed, but keep an eye out for worsening symptoms like more rampant dieback. No fungicides can reverse this damage or eliminate spores from the soil permanently. Nor would you want to attempt chemical soil sterilization, as plenty of beneficial fungi, bacteria, and other organisms inhabit the soil. Many of them can out-compete, directly kill or consume, or otherwise stifle the development of pathogens like these. Fungicide impacts are not limited to the pathogens, and some could even have harmful consequences for pollinators.
If Southern Blight is responsible, you can dig out and dispose of the affected plant, plus remove a few inches of the surface soil with it. This is not because the plant’s roots are going to also be infected, but because plant regrowth could be infected again by the spores remaining on the soil surface, and you don’t want to give the fungus more fuel for expansion. If you dig out soil but don’t want to throw it out (soil is a precious resource, after all), just bury the layers with potential spore contamination to a depth of 6 inches or so, where the spores should not survive.
Southern Blight, as with many similar blight and rot diseases, have an extensive range of plants they are capable of infecting. Why, then, don’t we see rampant die-offs in cultivated and wild plants for such common and destructive fungi? Interactions between organisms are complex, and as with insect diversity, there are microbe species which are beneficial, especially in healthy soil that is not too compacted, oxygen-deprived, wet or poorly drained, or low in organic matter (all conditions that favor common pathogens).
By Miri Talabac, Horticulturist, University of Maryland Extension Home & Garden Information Center. Miri writes the Garden Q&A for The Baltimore Sun and Washington Gardener Magazine. Read more by Miri.
Have a plant or insect question? The University of Maryland Extension has answers! Send your questions and photos to Ask Extension. Our horticulturists are available to answer your questions online, year-round.
Persistent wildfire smoke is new for Maryland gardeners. Experts seem to agree that smoke and ash do not pose a health risk for garden produce. Smoke diffuses sunlight but will probably not significantly reduce the total amount of light for photosynthesis. We have not heard/seen any reports of gardeners picking up smoky flavors in harvested greens or other vegetables or fruits.
Wash all produce prior to eating it raw or cooking with it
Wear an N-95 quality mask when working outside on days when wildfire smoke worsens air quality
Hose off plants if a noticeable soot layer develops from prolonged, intense smoke
Wildfire smoke has been shown to boost the levels of ozone and other air pollutants which can injure plants. Watermelon, squash, pumpkin, beans, and potato are especially vulnerable to high ozone levels (above 75 ppb).
Drought and damaging storms
Wildfire smoke interfered with weather patterns and likely contributed to cooler and drier weather across much of the state.
Mid-May through June:
Lower average temperatures
75% of state in moderate drought on July 3rd
Slow start for warm-season crops
July:
High heat and humidity
Spotty rainfall
Insect and disease issues increasing
The Maryland Department of the Environment announced a Drought Watch on July 10, encouraging voluntary reduction in residential water use.
Irregular brown spots and blotches appear on lilac leaves, followed by leaf curling and defoliation in late summer. Photo: UME Ask Extension
Q: My lilacs look like death-warmed-over this time of year. Do you know what’s wrong, and is there anything I can do at this point?
A: Lilacs are sadly not very well-suited to our mid-Atlantic conditions. We’re at the southern edge of their heat tolerance, so while they weren’t among the best flowering shrub choices to begin with, climate change is only going to worsen their prognosis. Several types of leaf-spotting fungi and bacteria, plus general heat stress (which also increases their vulnerability to borers), results in foliage that looks quite beat-up by late summer. Brown spots, crispy leaf edges, and bare stems from premature leaf drop are all typical. You can explore lilac ailments and their management on our lilac diagnostic page.
No fungicide will reverse these symptoms once they appear, and while they might work as a preventative if applied before bud-break (and re-applied repeatedly well into the summer), it’s simpler to just grow something else if a plant is going to be that much of a hassle. This is especially true if the treatments don’t work and the plant still winds up looking horrible. Fungicides also carry the risk of harming other organisms.
For now, you can rake up and dispose of any fallen leaves, though this isn’t a foolproof way of removing a source of infectious spores. Cut down the oldest, thickest stems this winter (they tend not to bloom well at that age anyway) and open up the canopy by selectively removing some stems that contribute to foliage crowding. You can do this thinning after bloom next spring.
For anyone really wanting to grow lilac despite these challenges, try cultivars with above-average disease resistance and heat tolerance. While not immune to problems, they perform much better, even if they don’t look exactly the same or have blooms as large or heavily perfumed. ‘Miss Kim’ is a round, compact-growing cultivar with pale lavender-purple flowers that’s been around for decades. Other varieties are now available with pink or deeper purple blooms, some of which even rebloom a bit, sporadically producing flowers into summer and early autumn, though high heat could still hamper that.
Dwarf lilac species and hybrids handle Maryland conditions much better than the traditional varieties. Some recent introductions will also re-bloom sporadically later in the summer. Photo: M. Talabac
All lilacs, but especially the traditional, classic “French” types, should be planted in a location with great air circulation (so, not up against a fence or wall) so wet leaves dry quickly after rain, dew, or irrigation. Wet foliage is more easily infected by pathogens.
The main perk of growing lilacs is fragrance, so if you want a scented replacement, consider: Winterhazel (Corylopsis), Koreanspice Viburnum (Viburnum carlesii) and its hybrids, Summersweet (Clethra), Seven-son Flower (Heptacodium), Carolina Allspice (Calycanthus), various deciduous Azaleas (Rhododendron viscosum and several others), Mockorange (Philadelphus), and Fragrant Abelia (Abelia mosanensis). Their scent characteristics, flower colors, mature sizes, and preferred growing conditions may differ from lilac, but nothing is going to be an exact substitute. Plus, several of these species will offer the additional bonus of showy autumn foliage or (for the native ones) better wildlife value. These are just some shrub ideas; there are also fragrant perennials and, if you have the room, several fragrant trees.
