LEGUME CRUCIFERE - BACTERIA

BACTERIAL LEAF SPOT OF CABBAGE

Causal organism: Pseudomonas syringae. pv. maculicola (no EPPO map available)

Host range: All brassica crops are commonly affected by bacterial leaf spots.

Occurrence and importance: Cabbage and cauliflower are most affected by Pseudomonas syringae. pv. Maculicola, the causal agent of bacterial leaf spots. This disease is important in coastal areas and is most severe during cool damp weather. An epidemic of the disease can be expected due to the increasing frequency of torrential rainfall in the European continent.

Symptoms and confusable diseases: Bacteriosis may attack cotyledons, stems, peduncles and leaves, pods and seeds. Initial symptoms can appear in the seedbed as small, angular to round, water-soaked spots, that later become brown and surrounded by a chlorotic halo. Lesions can join to form larger elongated areas. Severe seedling damage may result in plant death. Similar symptoms may occur in the field. In addition to the above symptoms, many lesions about 5-7 mm in diameter, surrounded by a water-soaked area may form on the undersides of leaves. These spots develop as a result of infection through the stomata. The spots later become dark brown (rarely purple) with a yellow halo. Infected leaves are distorted and puckered and fall off if severely affected. Such plants may be stunted and lack curds. Affected heads of cauliflower initially have dark brown necrotic spots. Under high humidity these spots may cover the head completely in two-three days. Damaged heads quickly decay and secondary organisms can invade the tissue.

Leaf spots can also result from Xanthomonas campestris pv. Armoraciae infection, but this pathogen is active in warmer temperatures. Symptoms include small dark brown spots on the leaves that spread into lesions. Their centers eventually degrade, resulting in circular holes in the leaves.

10. and 11. Figures: Symptoms of Pseudomonas syringae pv. maculicola (water-soaked spots on leaf, in left and dark brown necrotic spots on the cauliflower head, in right)

12. Figure: Symptoms of leaf spot, caused by Xanthomonas campestris pv. Armoraciae

Disease cycle: The pathogen can be primarily seed-borne in the field. Infection is most prevalent during cool and wet weather during vegetative growth and is occurs via the stomata or wounds caused by insects, frost or mechanical damage. Water is necessary for infection, it spreads in splashing water, including irrigation water. Pseudomonas syringae. pv. maculicola can overwinter in affected plant tissues and debris.

Control:

Bactericidal control is not available for leaf spot disease;
Planting disease-free seeds;
Hot water treatment of seeds;
Polythene-covered crops isolated from uncovered ones;
Irrigation practices – minimize the moisture that comes in contact with crucifer crops;
Crop rotation is critical in subsequent vegetation – the bacteria can overwinter in the soil;
Control of other pathogens and pests;
Resistance of cauliflower cultivars - should be exploited.

BACTERIAL SOFT ROT

Causal organism: Pectobacterium spp. (syn. Erwinia spp.); Pectobacterium carotovorum, Pectobacterium carotovorum ssp. atrosepticum, Pectobacterium versatile, Pectobacterium odoriferum and Pseudomonas cichorii and P. marginalis.

Host range: Most leaf, flower head and root-type brassica crops.

Occurrence and importance: Bacterial soft rot is one of the most damaging crop diseases worldwide. This disease can result in losses in several crops, including cabbages. The crops show symptoms in the field, but the pathogen can particularly damage cabbage, cauliflower, Chinese cabbage, kale, turnip, etc, during storage. Post-harvest losses have been estimated to vary between 15-30 per cent of the collected crops (Agrios, 2007).

Symptoms and confusable diseases: Symptoms of soft rots can appear on all plant parts, including leaves, internal of stems and root tissues. The initial signs of infection are small, water-soaked lesions that quickly enlarge and damage plant tissues and turn brown and mushy. In severe cases, the whole plant may decay entirely. Bacterial pathogens develop mainly under humid and warm weather conditions, nutritional deficiency and imbalance (high nitrogen application rates). Plants are easily infected because the soft rooting pathogens are common soil inhabitants. They can infect through wounds in plant tissue, including those caused by pests, frost and mechanical damage.

13. Figure: Symptoms of soft rots

Disease cycle: Soft-rot bacteria overwinters in the soil, in the field, greenhouse, on contaminated tools, containers, infected fresh tissues in storage, and in certain insects. The pathogens enter primarily through wounds made during planting, cultivation,, by pests, growth cracks, sunscald, freezing injuries, etc. Waterlogged and poorly-drained soil, rains and warm temperatures favour infection in the field, also high humidity in transit or storage. Plant surface moisture is essential for infection. Optimal temperature conditions are between 18° and 35° C; minimum temperatures are between 2° and 6° C and maximum between 35° and 41° C. The bacteria are killed at temperatures above 50° C.

Control:

No chemical control is available for soft rots
Resistant cabbage varieties
Prevent insect damage
Burn affected plants
Avoiding harvesting under wet weather conditions
Avoidance of high nitrogen levels is desirable
Where feasible, crop rotation with corn, grasses, alfalfa, small grains, or clovers
Biocontrol with endophytic bacterial strains
Avoid planting in areas with poorly drained soil, poor air circulation, overhead irrigation, overwatering (saturated soil) and excessive shade.

BLACK ROT

Causal organism: Xanthomonas campestris pv. Campestris

Host range: All cultivated Cruciferous species and some weeds.

Occurrence and importance: This disease occurs relatively frequently but often is neglected. Highest losses occur in cabbage and cauliflower. Commonly found in Europe.

Symptoms and confusable diseases: Typical symptoms occur on leaf margins as V-shaped spots yellow at the beginning, tan to brown and necrotized later. Black veins are often seen within these tan lesions, though they may not always develop. Severely infected leaves may wither and drop off. If systemic infection has taken place, the vascular tissues in petioles and main stems can also turn black. These symptoms can hardly be confused with symptoms caused by other pathogens. If temperatures are cool, however, symptoms may not be expressed. If necrotization affects several outer leaves, the inner head may rot. Severely infected young plants can die. Pods of seed plants can also be infected.

14. Figure: Xanthomonas campestris pv. campestris

15. Figure: Xanthomonas campestris pv. campestris

Disease cycle: Bacteria overwinters on crop debris and seeds. From seeds infected plants develop and bacteria spreads inside through vascular bundles. Infection from soil begins mainly from leaf margins where hydathodes serve as points of entry. Infection occurs during rain or irrigation when water droplets from soil reach the leaf surface. During colder nights water is exuded by hydathodes but during day it is reabsorbed together with bacteria. Warm days and colder nights thus favour disease spread. Many cruciferous weeds are important reservoirs of the pathogen.

Control:

crop rotation
elimination of Cruciferous weeds
deep ploughing can speed decomposition of infected plant debris
if possible, avoid overhead irrigation (but it may support some pests!)
some resistant cabbage varieties are available
seed disinfection (e.g. sodium hypochloride)

Listo of Viruses:

TURNIP MOSAIC VIRUS (TUMV), CAULIFLOWER MOSAIC VIRUS (CAMV), TURNIP YELLOW MOSAIC VIRUS (TYMV).

For more information on viruses, follow this link.

Listo of bacterial:

BACTERIAL LEAF SPOT OF CABBAGE
BACTERIAL SOFT ROT
BLACK ROT

For more information on the bacterium, follow this link.

Listo of fungi:

DAMPING OFF 10 DOWNY MILDEW
ALTERNARIA LEAF SPOT
VERTICILLIUM WILT
BLACK LEG
CLUBROOT

For more information on fungi, follow this link.

List of disorders:

EDEMA
HOLLOW STEM
LEAF EDGE BROWNING 19

For more information on disorders, follow this link.

List of insects:

FLEA BEETLES
CABBAGE ROOT FLY
CABBAGE APHID
LARGE AND SMALL WHITE BUTTERFLIES
MOTHS
DIAMONDBACK MOTH
ONION THRIPS
WEEVILS
CABBAGE CROWNGALL FLY (SWEDE MIDGE)
TURNIP SAWFLY
CABBAGE WHITEFLYFor more information on insects, follow this link.