Amaranth is a highly nutritious plant that is easy to grow and cultivate. However like any other plant it can be susceptible to various diseases that can affect its growth and yield. In this comprehensive guide, we will discuss the various amaranth plant diseases, their identification, and treatment methods.
Common Amaranth Plant Diseases
Some of the most common amaranth plant diseases include:
Downy Mildew
This fungal disease affects the leaves of amaranth plants It causes yellowish spots on the upper leaves, which then turn brown and die off. The undersides of the leaves develop a grayish mold that looks like a fine powder. This disease can spread quickly in humid conditions, making it important to identify and treat early
Root Rot
Root rot is caused by a fungus that attacks the roots of the amaranth plant. It causes the roots to become soft and mushy, making it difficult for the plant to absorb water and nutrients. This disease can be identified by the yellowing and wilting of the leaves, as well as the presence of blackened roots.
Leaf Spot
Leaf spot is a bacterial disease that affects the leaves of amaranth plants. It causes circular spots on the leaves that are dark brown or black and have a yellow halo around them. This disease can cause the leaves to wither and die off, leading to reduced yield.
Powdery Mildew
Powdery mildew is a fungal disease that affects the leaves of amaranth plants. It causes a white powdery coating on the leaves, which can spread rapidly in dry conditions. This disease can cause stunted growth, reduced yield, and even death in severe cases.
Anthracnose
This fungal disease causes necrotic lesions on the leaves and dieback of leaves and branches. It can be identified by the presence of brown or black spots on the stems and leaves.
Damping Off
Both fungal and bacterial damping off diseases affect amaranth seedlings. They cause poor germination, seedling collapse, lesions on stems near the soil, and failure of seedlings to emerge.
Choanephora Wet Rot
This fungal disease causes water-soaked lesions on stems that have a hairy appearance due to fungal spores. It can result in leaf loss and is spread via infected seeds, air, and mechanical damage.
Aphids
These tiny sap-sucking insects can quickly reproduce and cause yellowing leaves, stunted growth, and sticky residue on leaves.
Spider Mites
Tiny arachnids that feed on plant sap, causing yellow or brown spots and webbing on leaves and stems.
Cutworms
The larvae of moths that feed on stems of young plants, causing them to wilt and die.
Flea Beetles
Small jumping beetles that feed on leaves, causing small holes in the foliage.
Treatment of Amaranth Plant Diseases
The treatment of amaranth plant diseases depends on the specific disease affecting the plant:
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For downy mildew, leaf spot, and powdery mildew fungal diseases, remove and destroy infected plant material immediately. Improve air circulation around plants. Use fungicides containing copper, sulfur, potassium bicarbonate or neem oil.
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For root rot, improve drainage and remove infected plants. Use fungicides with trifloxystrobin or azoxystrobin.
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For bacterial leaf spot, remove and destroy infected leaves. Use bactericides containing copper or streptomycin sulfate.
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For damping off diseases, avoid overwatering and dense planting. Treat emerging diseases with fungicides.
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For choanephora wet rot, prevent wounds on plants. Remove infections and treat with copper fungicides.
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For insect pests like aphids and mites, use insecticidal soaps, neem oil, or introduce beneficial predators.
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For cutworms, handpick larvae from plants. Protect young plants with collars. Use Bacillus thuringiensis.
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For flea beetles, use row covers early on, handpick adults, or apply neem oil or diatomaceous earth.
Prevention of Amaranth Diseases
Preventing diseases is always better than treating them. Here are some tips:
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Start with disease-free seeds and seedlings
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Rotate amaranth crops yearly
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Remove weeds, infected plants, and crop debris
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Allow proper spacing and air circulation
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Avoid excess moisture on leaves
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Use row covers to exclude insect pests
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Apply balanced fertilizer to avoid excessive growth
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Scout regularly for early signs of diseases or pests
How to Identify Amaranth Plant Diseases
Identifying diseases early is crucial for effective treatment. Here are some signs to look for:
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Leaves: spots, lesions, wilting, yellowing, powdery or fuzzy coatings
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Stems and branches: lesions, rot, discoloration, dieback
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Roots: decay, mushiness, stunted growth
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Seedlings: poor germination, damping off, failure to emerge
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Whole plant: stunted growth, premature flowering or death
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Presence of pests: aphids, mites, caterpillars, beetles
Using Organic Methods to Treat Diseases
Many organic and natural methods can be used to prevent and treat amaranth diseases:
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Apply compost tea to suppress fungal diseases
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Use garlic oil or baking soda sprays to inhibit fungal growth
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Apply neem oil solution as a natural insecticide
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Introduce beneficial insects like ladybugs and lacewings to control pests
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Practice crop rotation with unrelated plants to break disease cycles
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Amend soil with compost to increase beneficial microbes
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Apply organic fungicides with sulfur or copper compounds
FAQs About Amaranth Diseases
Can amaranth diseases be prevented?
Yes, following good practices like crop rotation, sanitation, proper spacing and irrigation can help prevent many amaranth diseases.
Is it safe to eat diseased amaranth plants?
No, diseased plants may contain toxins or have reduced nutritional quality, so should not be consumed.
How often should I scout for diseases in my amaranth crop?
Aim to scout at least once a week during the growing season to catch diseases early before they spread or cause significant damage.
Can fungicides harm my amaranth plants?
Some harsh synthetic fungicides may harm amaranth. When possible, use organic-approved fungicides and test on a small area first.
What is the best way to remove diseased amaranth plants?
Carefully uproot diseased plants and place in sealed bags to dispose. Clean tools after use to avoid spreading disease.
Keys to Identifying Palmer Amaranth and Waterhemp
Palmer amaranth and waterhemp are two members of the pigweed family that are listed as noxious weeds by the Code of Maryland Regulations (15.06.05.03). Their status as noxious weeds requires Maryland landowners to manage these species on all types of land. These two species are resistant to multiple herbicide groups, and are well adapted to conditions in the Mid-Atlantic area. Once established in agronomic and vegetable fields they can be very difficult to control. Therefore, early identification is especially important. Three other pigweed species that are commonly found in Maryland – spiny amaranth, smooth pigweed, and redroot pigweed – are similar in appearance to Palmer amaranth and waterhemp. These species also pose problems in agronomic and vegetable crops, but their control is not mandatory. Being able to accurately identify these five species will help landowners with their management efforts.
The following botanical characteristics will help differentiate the five species listed above in the field:
- stems,
- leaves, and
- flowers
Look for the presence of small hairs on the stem of the plant you need to identify. The stems of Palmer amaranth, waterhemp, and spiny amaranth do not have hairs (Figure 1). Smooth pigweed and redroot pigweed have visible hairs even when the plants are about two inches tall (Figure 2).
Figure 2. Young Palmer amaranth (a) and smooth pigweed (b). Note the presence of stem hairs on smooth pigweed. Photo credit: Mark VanGessel, University of Delaware.
Figure 3. Leaves of Palmer amaranth waterhemp and smooth pigweed (a). Palmer amaranth petioles are often as long as or longer than the leaf itself (b). Photo credit: Kurt Vollmer, University of Maryland.
Compare the length of the leaf petiole (the stalk that attaches the leaf blade to the stem) to the length of the leaf blade. Mature leaves of Palmer amaranth will have longer petioles than their respective leaf blades when compared to those of other pigweed species (Figure 3). When folded over, the petiole of mature Palmer amaranth leaves will be as long as or longer than the leaf blade itself. Note: longer petioles can also be a feature of spiny amaranth; be sure to look for spiny amaranth’s distinctive (0.2-0.4 in.) spine at the base of the leaf (Figure 4).
Figure 4. Spines at the base of the stems on spiny amaranth. Photo credit: John D. Byrd, Mississippi State University, Bugwood.org.
Figure 5. Leaves of Palmer amaranth (a), smooth pigweed (b), and waterhemp (c). Photo credit: Kurt Vollmer, University of Maryland.
These species are also be differentiated by their leaf shape. Compared to other pigweed species, waterhemp leaves are long and linear (Figure 5). Palmer and spiny amaranth leaves are typically egg- to diamond-shaped, and redroot and smooth pigweed leaves are oval- to egg-shaped. Redroot and smooth pigweed leaves will also have wavy edges (Figure 6). Palmer amaranth leaves have no hairs on them, but the presence of a single hair in the notch of the leaf tip of some Palmer amaranth plants may be used to help differentiate this species from waterhemp (Figure 7). Note: a V-shaped watermark is often found on Palmer amaranth leaves, but this feature can also be present occasionally on redroot pigweed and spiny amaranth (Figure 8).
Figure 6. The leaves of smooth pigweed (a) typically have a wavy appearance compared to Palmer amaranth (b) and waterhemp (c). Photo credit: Kurt Vollmer, University of Maryland.
Figure 7. Palmer amaranth with a single hair in the notch of the leaf tip. Photo credit: Kurt Vollmer, University of Maryland.
Figure 8. Watermarks on leaves of Palmer amaranth (a), redroot pigweed (b), and spiny amaranth (c). Photo credit: Kurt Vollmer, University of Maryland.
Flowers Figure Terminal flower spikes of Palmer amaranth (a) are typically longer than those of other pigweed species, such as redroot pigweed (b). Photo credit: Ben Beale and Kurt Vollmer, University of Maryland.
Palmer amaranth plants produce the longest flower spikes among these five pigweed species (2 to 3 ft.; see Figure 9). Palmer amaranth and waterhemp have separate male and female plants. Male plants only produce pollen and female plants only produce seed. The seed-producing female flowers of Palmer amaranth are tightly clustered and contain sharp bracts (0.12-0.24 in.) at the base of the flowers located in the leaf axils, whereas male flowers can appear yellow, are soft, and will release pollen when touched. Female waterhemp flowers do not have sharp bracts and are less tightly clustered than that of Palmer amaranth. In contrast, spiny amaranth, smooth pigweed, and redroot pigweed contain both male and female flowers of on the same plant. Male spiny amaranth flowers can be found at the terminal spike, and female flowers are located on the stem axils. Male and female flowers of both redroot and smooth pigweed are produced in the terminal flower spikes. The flower spikes of redroot pigweed will have a short, stubby appearance, whereas smooth pigweed flower spikes are more slender and less compact.