Broccoli, Brassica oleracea, is an herbaceous annual or biennial grown for its edible flower heads which are used as a vegetable. The broccoli plant has a thick green stalk, or stem, which gives rise to thick, leathery, oblong leaves which are gray-blue to green in color. The plant produces large branching green flower heads covered with numerous white or yellow flowers. Broccoli can be annual or biennial depending on the variety and can grow to 1 m (3.3 ft) in height. Broccoli may also be referred to as sprouting broccoli and likely originates from the Mediterranean although the exact location has not been determined.

Common Pests and Diseases That Affect Broccoli Farming In Kenya

Broccoli is a cool-season vegetable that can be grown in Kenya during the spring and fall. It is a relatively easy crop to grow, and it can be harvested in about 70-80 days.

Choose a planting site

Broccoli prefers full sun and well-drained soil. It is important to avoid planting broccoli in areas where water tends to pool. If your soil is heavy clay, you may need to amend it with sand or compost to improve drainage.

Prepare the soil

Before planting, loosen the soil to a depth of 12-20 inches. Add a 2-3 inch layer of compost or manure to the soil. You can also add a balanced fertilizer, such as 10-10-10.

Plant the broccoli

Broccoli can be planted from seed or from transplants. If you are planting from seed, sow the seeds directly in the ground 4-6 weeks before the last frost date. If you are planting from transplants, plant them 2-3 weeks before the last frost date.

Space the broccoli plants 12-18 inches apart in rows that are 24-36 inches apart.

Water and fertilize the broccoli

Broccoli needs regular watering, especially during hot weather. Water the broccoli deeply once a week, or more often if the weather is hot and dry.

Fertilize the broccoli every 2-3 weeks with a balanced fertilizer, such as 10-10-10. You can also use a fertilizer that is specifically designed for broccoli.

Harvest the broccoli

Broccoli is ready to harvest when the heads are tightly closed and firm. The heads should be about 6-8 inches in diameter.

To harvest the broccoli, cut the head off of the plant with a sharp knife. Be sure to leave about 2 inches of stem attached to the head.

Harvesting broccoli multiple times

Some broccoli varieties can be harvested multiple times. To do this, harvest the outer heads of the plant as they mature. This will encourage the plant to produce more side shoots.

Problems Faced By Broccoli Farmers

There are two basic types of problems with any vegetable, including Broccoli: pest problems and health problems. Broccoli is no exception. If you are losing plants at any growth stage, it is good to find out whether the problem is a pest or a problem with the plant’s health. A lot of pests visit the Brassicas. Humans have been growing Broccoli, Cabbage, and other members of the Brassicas family for thousands of years.

Some pests have become as attached to these hearty, nutritious vegetables. Some fungi and diseases can destroy the Broccoli crop as well. But if you are paying attention and taking preventive measures, it will be easy to keep your Broccoli crop healthy and grow stronger.

Common Pests and Diseases of broccoli

Diseases

Category : Fungal

Alternaria leaf spot Alternaria brassicae

Symptoms
Small dark spots on leaves which turn brown to gray; lesions may be round or angular and may possess a purple-black margin; lesions may form concentric rings, become brittle and crack in center; dark brown elongated lesions may develop on stems and petioles
Cause
Fungus
Comments
Disease emergence favored by warm, rainy weather
Management

Plant only pathogen-free seed; rotate crops; applications of appropriate fungicides control disease when present

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Black rot Xanthomonas campestris

Symptoms
Irregularly shaped dull yellow areas along leaf margins which expand to leaf midrib and create a characterstic “V-shaped” lesion; lesions may coalesce along the leaf margin to give plant a scorched appearance
Cause
Bacteria
Comments
Pathogen is spread via infected seed or by splashing water and insect movement; disease emergence favored by warm and humid conditions
Management

Primary method of controlling black rot is through the use of good sanitation practices; rotate crops to non-cruciferous crops every 2 years; plant resistant varieties; control cruciferous weed species which may act as a reservoir for bacteria; plant pathogen-free seed

Clubroot Plasmodiophora brassicae

Symptoms
Slow growing, stunted plants; yellowish leaves which wilt during day and rejuvenate in part at night; swollen, distorted roots; extensive gall formation
Cause
Fungus
Comments
Can be difficult to distinguish from nematode damage; fungus can survive in soil for periods in excess of 10 years; can be spread by movement of contaminated soil and irrigation water to uninfected areas
Management

Once the pathogen is present in the soil it can survive for many years, elimination of the pathogen is economically unfeasible; rotating crops generally does not provide effective control; plant only certified seed and avoid field grown transplants unless produced in a fumigated bed; applying lime to the soil can reduce fungus sporulation

Powdery mildew Erysiphe cruciferarum

Symptoms
Small white patches on upper and lower leaf surfaces which may also show purple blotching; patches coalesce to form a dense powdery layer which coats the leaves; leaves become chlorotic and drop from plant
Cause
Fungus
Comments
Disease emergence favored by dry season, moderate temperatures, low humidity and low levels of rainfall
Management

Plant resistant varieties; rotate crops; remove all crop debris after harvest; remove weeds; avoid excessive application of nitrogen fertilizer which encourages powdery mildew growth; powdery mildew can be controled by application of sulfur sprays, dusts or vapors

Sclerotinia stem rot (White mold) Sclerotinia sclerotiorum

Symptoms
Irregular, necrotic gray lesions on leaves; white-gray leions on stems; reduced pod set; shattering seed pods
Cause
Fungus
Comments
Disease emergence favors moderate to cool temperatures and high humidity
Management

Rotate crop to non-hosts (e.g. cereals) for at least 3 years; control weeds; avoid dense growth by planting in adequately spaced rows; apply appropriate foliar fungicides

White rust Albugo candida

Symptoms
White pustules on cotyledons, leaves, stems and/or flowers which coalesce to form large areas of infection; leaves may roll and thicken
Cause
Fungus
Comments
Fungus can survive for long periods of time in dry conditions; disease spread by wind
Management

Rotate crops; plant only disease-free seed; apply appropriate fungicide if disease becomes a problem

Category : Bacterial

Black leg Phoma lingam

Symptoms
Damping-off of seedlings; round or irregularly shaped gray necrotic lesions on leaves with dark margins; lesions may be covered in pink masses in favorable weather conditions
Cause
Fungus
Comments
Favors warm, wet conditions; higher temperatures result in the development of more visible symptoms
Management

Use disease free seed or treat with hot water to remove fungus prior to planting; remove and destroy crop debris after harvest or plow deeply into soil

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Category : Fungal, Oomycete

Downy mildew Hyaloperonospora parasitica

Symptoms
Small angular lesions on upper surface of leaves which enlarge into orange or yellow necrotic patches; white fluffy growth on undersides of leaves
Cause
Fungus
Comments
Disease emergence favored by cool, moist conditions
Management

Remove all crop debris after harvest; rotate with non-brassicas; it is possible to control downy mildew with the application of an appropriate fungicide

Wirestem (Damping-off) Rhizoctonia solani

Symptoms
Death of seedlings after germination; brown-red or black rot girdling stem; seedling may remain upright but stem is constricted and twisted (wirestem)
Cause
Fungus
Comments
Disease emergence in seedlings favored by cool temperatures
Management

Plant pathogen-free seed or transplants that have been produced in sterilized soil; apply fungicide to seed to kill off any fungi; shallow plant seeds or delay planting until soil warms

Category : Viral

Ring spot Mycosphaerella brassicicola

Symptoms
Small, purple spots surrounded by a ring of water-soaked tissue on leaves which mature to brown spots with olive green borders 1-2 cm across; spots may develop numerous fruiting bodies which give them a black appearance or develop a concentric pattern; heavily infected leaves may dry up and curl inwards
Cause
Fungus
Comments
Ring spot requires cool, moist conditions to survive; disease symptoms typically develop in the fall and the peak of the infection occurs in winter
Management

Refrain from planting in areas known to have had disease previously; rotate crop to non-brassicas; sanitize tools and equipment regularly; apply appropriate fungicide if disease is identified in crop

Pests

Category : Insects

Diamondback moth Plutella xylostella

Symptoms
Young larvae feed between upper and lower leaf surface and may be visible when they emerge from small holes on the underside of the leaf; older larvae leave large, irregularly shaped shotholes on leaf undersides, may leave the upper surface intact; larvae may drop from the plant on silk threads if the leaf is disturbed; larvae are small (1 cm/0.3 in) and tapered at both ends; larvae have to prolegs at the rear end that are arranged in a distinctive V-shape
Cause
Insect
Comments
Larvae take between 10 and 14 days to mature and spin a loose, gauze-like cocoon on leaves or stems to pupate
Management

Larvae can be controlled organically by applications of Bacillus thurengiensis or Entrust; application of appropriate chemical insecticide is only necessary if larvae are damaging the growing tips of the plants

Flea beetle (Crucifer flea beetle) Phyllotreta cruciferae

Symptoms
Small holes or pits in leaves that give the foliage a characteristic “shothole” appearance; young plants and seedlings are particularly susceptible; plant growth may be reduced; if damage is severe the plant may be killed; the pest responsible for the damage is a small (1.5–3.0 mm) dark colored beetle which jumps when disturbed; the beetles are often shiny in appearance
Cause
Insect
Comments
Younger plants are more susceptible to flea beetle damage than older ones; older plants can tolerate infestation; flea beetles may overwinter on nearby weed species, in plant debris or in the soil; insects may go through a second or third generation in one year
Management

In areas where flea beetles are a problem, floating row covers may have to be used prior to the emergence of the beetles to provide a physical barrier to protect young plants; plant seeds early to allow establishment before the beetles become a problem – mature plants are less susceptible to damage; trap crops may provide a measure of control – cruciferous plants are best; application of a thick layer of mulch may help prevent beetles reaching surface; application on diamotecoeus earth or oils such as neem oil are effective control methods for organic growers; application of insecticides containing carbaryl, spinosad, bifenthrin and permethrin can provide adequate control of beetles for up to a week but will need reapplied

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Large cabbage white (Cabbageworm) Pieres rapae

Symptoms
Large ragged holes in leaves; green-brown frass (insect feces) on leaves; caterpillar is green in color and hairy, with a velvet-like appearance; may have faint yellow to orange stripes down back; slow-moving compared with other caterpillars
Cause
Insect
Comments
Butterfly larvae cause damage by feeding on plants; can be distinguished from other caterpillars by its sluggish movement; in large numbers larvae can cause extensive damage very quickly
Management

Hand pick caterpillars from plants and destroy; scrape eggs from leaves prior to hatching; apply appropriate insecticide if infestation is very heavy

Thrips (Western flower thrips, Onion thrips, etc.) Frankliniella occidentalis
Thrips tabaci

Symptoms
If population is high leaves may be distorted; leaves are covered in coarse stippling and may appear silvery; leaves speckled with black feces; insect is small (1.5 mm) and slender and best viewed using a hand lens; adult thrips are pale yellow to light brown and the nymphs are smaller and lighter in color
Cause
Insect
Comments
Transmit viruses such as Tomato spotted wilt virus; once acquired, the insect retains the ability to transmit the virus for the remainder of its life
Management

Avoid planting next to onions, garlic or cereals where very large numbers of thrips can build up; use reflective mulches early in growing season to deter thrips; apply appropriate insecticide if thrips become problematic

Category : Nematodes

Root knot nematode Meloidogyne spp.

Symptoms
Galls on roots which can be up to 3.3 cm (1 in) in diameter but are usually smaller; reduction in plant vigor; yellowing plants which wilt in hot weather
Cause
Nematode
Comments
Galls can appear as quickly as a month prior to planting; nematodes prefer sandy soils and damage in areas of field or garden with this type of soil is most likely
Management

Plant resistant varieties if nematodes are known to be present in the soil ;check roots of plants mid-season or sooner if symptoms indicate nematodes; solarizing soil can reduce nematode populations in the soil and levels of inoculum of many other pathogens

Reference: https://plantvillage.psu.edu/topics/broccoli/infos

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