Spring Seeding Guide - Disease and Predation
Hereabouts, recently, the most common forms of seedling disease are ‘damping off’, mildew, and aphid infestation. Flowers seem more disease-prone than veggies.
Damping off is not one disease but several similar ones. The first sign of it comes when a few seedlings out of a large group collapse. Brassicas, toms and peppers seem most susceptible. The green leaves are still intact, but the base of the stems at the soil line are dark and have rotted. Remove the affected seedlings and examine those around them. If a tray is affected, quarantine it.
Damping off is a typical symptom of too-crowded seedlings/poor air circulation and, most likely, overwatering. If the disease appears systemic, your soil mix is probably a factor. Sphagnum moss is a natural damping-off agent.
Removing affected plants, improving air circulation, and adjusting watering practices usually suffices to stop the problem in its tracks.
Downy mildew appears on the underside of leaves, usually lettuce or Brassicas. (It is powdery mildew we see on cucurbits.) Compost-tea sprays work well to prevent and address the problem. Again, it is a disease associated with excessive moisture or humidity. The disease is spore-driven and can spread quickly. Quarantine or compost affected plants.
Aphids multiply furiously, but are delicate creatures highly susceptible to water/soap/organic sprays. One or two applications will usually suffice to settle an outbreak.
Slugs and snails are by far the most dangerous predators in urban environments where threats from mice and birds during the hungry months of spring appear less prevalent. When seedlings are small, a large snail on a greenhouse bench can wreak devastation in a single night, wiping out entire sowings. Slugs and snails have an uncanny ability to zero in on your most precious, difficult-to-germinate seedlings (last year, a rare plant which took me over a year to germ, was the very first slug-meal of the season).
Keeping the gastropods at bay in the first place is your best bet. Each of the legs of my greenhouse tables is wrapped in a 3" wide piece of copper strip. The slugs will not cross it. Copper wire will not do. Copper strip is expensive but, given the challenges of dealing with slugs once they make it onto my greenhouse table, this barrier represents a major investment in peace of mind. If going the barrier route, ensure that no potential bridges across any 'line of death' are touching the bench beyond it - such as plants growing up underneath. If you have electrical cords running to your benches, keep an eye on them. Likewise, whenever you move trays and pots onto your greenhouse benches, make a habit of checking to see that no tourists are hitching a ride. A favored gastropod Trojan Horse, that one. I am told that slugs will descend upon a bench from a slime-rope attached to an overhead surface, such as the curving roof of a greehouse, but I have not seen this.
with your plants will reveal the presence of any interloper. Once cursing
slows, lifting and examining pots and trays in the immediate vicinity
of the damage will often reveal the offender. Sometimes, a particularly
frustrating character will evade detection for days. A nighttime visit,
flashlight in hand, may then be necessary to target the dinner-guest.
By Nick Routledge for the Observer Allotment Blog